Experimental Group – Definition, Importance, Examples

What is experimental group, definition of experimental group.

The experimental group, in scientific research, refers to the group subjected to specific changes or treatments in a variable to observe and evaluate potential outcomes, in contrast to a control group which remains unaltered or standardized for comparison.

How Does an Experimental Group Work?

In essence, the experimental group serves as the primary focus of investigation in experimental research. By comparing its outcomes with a control group, researchers can isolate the effects of the treatment or intervention, thereby providing insights into cause-and-effect relationships.

Advantages of Experimental Group

In summary, the use of an experimental group in research provides a structured and controlled environment that facilitates the exploration of cause-and-effect relationships, ensuring the rigor, validity, and reliability of the findings.

Limitation of Experimental Group

In summary, while experimental groups provide invaluable insights in scientific research, it’s crucial to recognize and account for these limitations when designing studies, interpreting results, and drawing conclusions.

Importance of Experimental Group

In summary, the experimental group is indispensable in the scientific method, providing a structured framework for investigating hypotheses and drawing informed conclusions. Its role in isolating and examining the effects of specific variables ensures the rigor and robustness of empirical research.

Examples of Experimental Group

1. the influence of music on plant growth.

In a structured experiment designed to investigate the potential impact of music on plant growth, plants were systematically categorized into distinct groups. The primary division comprised the control group, which was maintained in an environment devoid of music, and the experimental group. This experimental group was further subdivided, with each subset being exposed to a unique genre of music. To ensure the reliability of the results, it was imperative that the plants used were genetically identical, ideally clones, and that they exhibited homozygosity across all genes. This minimizes genetic variability, which could otherwise confound the results. By meticulously controlling external factors such as temperature and humidity, the experiment aimed to ascertain that any observed differences in growth patterns were solely attributable to the musical variations.

2. Ecosystem Productivity and Organism Interactions

In essence, these examples underscore the pivotal role of experimental groups in scientific research. By introducing specific changes or treatments to these groups and juxtaposing their outcomes against control groups, scientists can glean invaluable insights into the variables under investigation.

What is the primary purpose of an experimental group in scientific research? a) To serve as a baseline for comparison b) To undergo specific treatments or interventions c) To ensure the study has high external validity d) To replicate the results of previous studies

In an experimental design, which group remains unaltered or is exposed to a baseline level of the variable for comparison? a) Dependent group b) Independent group c) Experimental group d) Control group

Which of the following is a potential limitation of using an experimental group? a) High internal validity b) Lack of generalizability to real-world settings c) Ability to establish causality d) Precision in measurements

The Hawthorne Effect is associated with which phenomenon? a) Participants altering their behavior due to the experimental treatment b) Participants behaving differently because they know they are being observed c) Participants responding positively to any intervention d) Participants showing no change regardless of the intervention

Why is random assignment important in experimental research? a) To ensure high external validity b) To reduce the likelihood of selection bias c) To increase the sample size d) To ensure all participants receive the treatment

Which of the following best describes the control group in an experimental design? a) It undergoes the primary treatment or intervention. b) It is exposed to multiple variables simultaneously. c) It remains unexposed or is exposed to a standard level of the variable. d) It determines the external validity of the study.

In experimental research, what is the primary goal of controlling extraneous variables? a) To increase the complexity of the study b) To ensure that observed effects are due to the manipulated variable c) To reduce the sample size d) To enhance external validity

Which of the following is NOT a typical characteristic of an experimental group? a) Undergoing a specific treatment or intervention b) Serving as a benchmark for comparison c) Being observed for outcomes post-treatment d) Being randomly selected from the larger sample

Experimental groups are essential for: a) Qualitative research only b) Establishing correlational relationships c) Establishing cause-and-effect relationships d) Observational studies

Which of the following scenarios best exemplifies the use of an experimental group? a) Observing the natural behavior of animals in the wild b) Conducting a survey on people’s dietary habits c) Administering a new drug to a group of patients to test its efficacy d) Interviewing individuals about their life experiences

What is an experimental group in scientific research?

How is an experimental group different from a control group.

While the experimental group undergoes the treatment or intervention, the control group remains unaltered or is exposed to a baseline level of the variable. The control group serves as a benchmark for comparison.

Why is random assignment important in creating an experimental group?

Random assignment ensures that each participant has an equal chance of being placed in any group, reducing potential biases and ensuring that the groups are comparable at the outset.

Can a study have multiple experimental groups?

What is the main purpose of using an experimental group.

The primary purpose is to determine the effects of a specific treatment or intervention by comparing the outcomes of the experimental group to those of a control group.

How do researchers ensure that results from the experimental group are valid?

Researchers control extraneous variables, use random assignment, and employ statistical tests to ensure that observed effects can be attributed to the treatment rather than other factors.

What are some limitations of using an experimental group?

Is the experimental group always exposed to positive interventions.

No, the experimental group can be exposed to any type of intervention, whether it’s believed to have positive, negative, or neutral effects.

How do researchers handle potential biases in experimental groups?

Through techniques like random assignment, blinding (where participants or researchers don’t know who is receiving the treatment), and controlling extraneous variables.

Can experimental groups be used in fields outside of medicine or biology?

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The Difference Between Control Group and Experimental Group

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In an experiment , data from an experimental group is compared with data from a control group. These two groups should be identical in every respect except one: the difference between a control group and an experimental group is that the independent variable is changed for the experimental group, but is held constant in the control group.

Key Takeaways: Control vs. Experimental Group

  • The control group and experimental group are compared against each other in an experiment. The only difference between the two groups is that the independent variable is changed in the experimental group. The independent variable is "controlled", or held constant, in the control group.
  • A single experiment may include multiple experimental groups, which may all be compared against the control group.
  • The purpose of having a control is to rule out other factors which may influence the results of an experiment. Not all experiments include a control group, but those that do are called "controlled experiments."
  • A placebo may also be used in an experiment. A placebo isn't a substitute for a control group because subjects exposed to a placebo may experience effects from the belief they are being tested; this itself is known as the placebo effect.

What Are Is an Experimental Group in Experiment Design?

An experimental group is a test sample or the group that receives an experimental procedure. This group is exposed to changes in the independent variable being tested. The values of the independent variable and the impact on the dependent variable are recorded. An experiment may include multiple experimental groups at one time.

A control group is a group separated from the rest of the experiment such that the independent variable being tested cannot influence the results. This isolates the independent variable's effects on the experiment and can help rule out alternative explanations of the experimental results.

While all experiments have an experimental group, not all experiments require a control group. Controls are extremely useful where the experimental conditions are complex and difficult to isolate. Experiments that use control groups are called controlled experiments .

A Simple Example of a Controlled Experiment

A simple example of a controlled experiment may be used to determine whether or not plants need to be watered to live. The control group would be plants that are not watered. The experimental group would consist of plants that receive water. A clever scientist would wonder whether too much watering might kill the plants and would set up several experimental groups, each receiving a different amount of water.

Sometimes setting up a controlled experiment can be confusing. For example, a scientist may wonder whether or not a species of bacteria needs oxygen in order to live. To test this, cultures of bacteria may be left in the air, while other cultures are placed in a sealed container of nitrogen (the most common component of air) or deoxygenated air (which likely contained extra carbon dioxide). Which container is the control? Which is the experimental group?

Control Groups and Placebos

The most common type of control group is one held at ordinary conditions so it doesn't experience a changing variable. For example, If you want to explore the effect of salt on plant growth, the control group would be a set of plants not exposed to salt, while the experimental group would receive the salt treatment. If you want to test whether the duration of light exposure affects fish reproduction, the control group would be exposed to a "normal" number of hours of light, while the duration would change for the experimental group.

Experiments involving human subjects can be much more complex. If you're testing whether a drug is effective or not, for example, members of a control group may expect they will not be unaffected. To prevent skewing the results, a placebo may be used. A placebo is a substance that doesn't contain an active therapeutic agent. If a control group takes a placebo, participants don't know whether they are being treated or not, so they have the same expectations as members of the experimental group.

However, there is also the placebo effect to consider. Here, the recipient of the placebo experiences an effect or improvement because she believes there should be an effect. Another concern with a placebo is that it's not always easy to formulate one that truly free of active ingredients. For example, if a sugar pill is given as a placebo, there's a chance the sugar will affect the outcome of the experiment.

Positive and Negative Controls

Positive and negative controls are two other types of control groups:

  • Positive control groups are control groups in which the conditions guarantee a positive result. Positive control groups are effective to show the experiment is functioning as planned.
  • Negative control groups are control groups in which conditions produce a negative outcome. Negative control groups help identify outside influences which may be present that were not unaccounted for, such as contaminants.
  • Bailey, R. A. (2008). Design of Comparative Experiments . Cambridge University Press. ISBN 978-0-521-68357-9.
  • Chaplin, S. (2006). "The placebo response: an important part of treatment". Prescriber : 16–22. doi: 10.1002/psb.344
  • Hinkelmann, Klaus; Kempthorne, Oscar (2008). Design and Analysis of Experiments, Volume I: Introduction to Experimental Design (2nd ed.). Wiley. ISBN 978-0-471-72756-9.
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Experimental Group

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As noted in the Experimental Designs entry, experimental designs are those in which the influence(s) of interest are controlled by the research scientist. An experimental group is a group of subjects who receive a particular treatment or intervention. Experimental subjects are randomly assigned to one of the treatment groups so that many potential influences that cannot be controlled for (e.g., sex, height, and weight) are likely to be as frequent in one experimental group as they are in the other.

It should be noted that the term “treatment group” is related to the term “experimental group,” but they are not synonymous. Experimental groups can be thought of as a subset of treatment groups, i.e., groups formed by research scientists before administering the treatment or intervention of interest. Treatment groups can be formed retrospectively. For example, a research scientist may wish to collect follow-up data for patients who received two kinds of intervention for the same...

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References and Readings

Piantadosi, S. (2005). Clinical trials: A methodologic perspective (2nd ed.). Hoboken, NJ: Wiley.

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Turner, J.R. (2013). Experimental Group. In: Gellman, M.D., Turner, J.R. (eds) Encyclopedia of Behavioral Medicine. Springer, New York, NY. https://doi.org/10.1007/978-1-4419-1005-9_1023

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Experimental vs Observational Studies: Differences & Examples

Experimental vs Observational Studies: Differences & Examples

Understanding the differences between experimental vs observational studies is crucial for interpreting findings and drawing valid conclusions. Both methodologies are used extensively in various fields, including medicine, social sciences, and environmental studies. 

Researchers often use observational and experimental studies to gather comprehensive data and draw robust conclusions about their investigating phenomena. 

This blog post will explore what makes these two types of studies unique, their fundamental differences, and examples to illustrate their applications.

What is an Experimental Study?

An experimental study is a research design in which the investigator actively manipulates one or more variables to observe their effect on another variable. This type of study often takes place in a controlled environment, which allows researchers to establish cause-and-effect relationships.

Key Characteristics of Experimental Studies:

  • Manipulation: Researchers manipulate the independent variable(s).
  • Control: Other variables are kept constant to isolate the effect of the independent variable.
  • Randomization: Subjects are randomly assigned to different groups to minimize bias.
  • Replication: The study can be replicated to verify results.

Types of Experimental Study

  • Laboratory Experiments: Conducted in a controlled environment where variables can be precisely controlled.
  • Field Research : These are conducted in a natural setting but still involve manipulation and control of variables.
  • Clinical Trials: Used in medical research and the healthcare industry to test the efficacy of new treatments or drugs.

Example of an Experimental Study:

Imagine a study to test the effectiveness of a new drug for reducing blood pressure. Researchers would:

  • Randomly assign participants to two groups: receiving the drug and receiving a placebo.
  • Ensure that participants do not know their group (double-blind procedure).
  • Measure blood pressure before and after the intervention.
  • Compare the changes in blood pressure between the two groups to determine the drug’s effectiveness.

What is an Observational Study?

An observational study is a research design in which the investigator observes subjects and measures variables without intervening or manipulating the study environment. This type of study is often used when manipulating impractical or unethical variables.

Key Characteristics of Observational Studies:

  • No Manipulation: Researchers do not manipulate the independent variable.
  • Natural Setting: Observations are made in a natural environment.
  • Causation Limitations: It is difficult to establish cause-and-effect relationships due to the need for more control over variables.
  • Descriptive: Often used to describe characteristics or outcomes.

Types of Observational Studies: 

  • Cohort Studies : Follow a control group of people over time to observe the development of outcomes.
  • Case-Control Studies: Compare individuals with a specific outcome (cases) to those without (controls) to identify factors that might contribute to the outcome.
  • Cross-Sectional Studies : Collect data from a population at a single point to analyze the prevalence of an outcome or characteristic.

Example of an Observational Study:

Consider a study examining the relationship between smoking and lung cancer. Researchers would:

  • Identify a cohort of smokers and non-smokers.
  • Follow both groups over time to record incidences of lung cancer.
  • Analyze the data to observe any differences in cancer rates between smokers and non-smokers.

Difference Between Experimental vs Observational Studies

TopicExperimental StudiesObservational Studies
ManipulationYesNo
ControlHigh control over variablesLittle to no control over variables
RandomizationYes, often, random assignment of subjectsNo random assignment
EnvironmentControlled or laboratory settingsNatural or real-world settings
CausationCan establish causationCan identify correlations, not causation
Ethics and PracticalityMay involve ethical concerns and be impracticalMore ethical and practical in many cases
Cost and TimeOften more expensive and time-consumingGenerally less costly and faster

Choosing Between Experimental and Observational Studies

The researchers relied on statistical analysis to interpret the results of randomized controlled trials, building upon the foundations established by prior research.

Use Experimental Studies When:

  • Causality is Important: If determining a cause-and-effect relationship is crucial, experimental studies are the way to go.
  • Variables Can Be Controlled: When you can manipulate and control the variables in a lab or controlled setting, experimental studies are suitable.
  • Randomization is Possible: When random assignment of subjects is feasible and ethical, experimental designs are appropriate.

Use Observational Studies When:

  • Ethical Concerns Exist: If manipulating variables is unethical, such as exposing individuals to harmful substances, observational studies are necessary.
  • Practical Constraints Apply: When experimental studies are impractical due to cost or logistics, observational studies can be a viable alternative.
  • Natural Settings Are Required: If studying phenomena in their natural environment is essential, observational studies are the right choice.

Strengths and Limitations

Experimental studies.

  • Establish Causality: Experimental studies can establish causal relationships between variables by controlling and using randomization.
  • Control Over Confounding Variables: The controlled environment allows researchers to minimize the influence of external variables that might skew results.
  • Repeatability: Experiments can often be repeated to verify results and ensure consistency.

Limitations:

  • Ethical Concerns: Manipulating variables may be unethical in certain situations, such as exposing individuals to harmful conditions.
  • Artificial Environment: The controlled setting may not reflect real-world conditions, potentially affecting the generalizability of results.
  • Cost and Complexity: Experimental studies can be costly and logistically complex, especially with large sample sizes.

Observational Studies

  • Real-World Insights: Observational studies provide valuable insights into how variables interact in natural settings.
  • Ethical and Practical: These studies avoid ethical concerns associated with manipulation and can be more practical regarding cost and time.
  • Diverse Applications: Observational studies can be used in various fields and situations where experiments are not feasible.
  • Lack of Causality: It’s easier to establish causation with manipulation, and results are limited to identifying correlations.
  • Potential for Confounding: Uncontrolled external variables may influence the results, leading to biased conclusions.
  • Observer Bias: Researchers may unintentionally influence outcomes through their expectations or interpretations of data.

Examples in Various Fields

  • Experimental Study: Clinical trials testing the effectiveness of a new drug against a placebo to determine its impact on patient recovery.
  • Observational Study: Studying the dietary habits of different populations to identify potential links between nutrition and disease prevalence.
  • Experimental Study: Conducting a lab experiment to test the effect of sleep deprivation on cognitive performance by controlling sleep hours and measuring test scores.
  • Observational Study: Observing social interactions in a public setting to explore natural communication patterns without intervention.

Environmental Science

  • Experimental Study: Testing the impact of a specific pollutant on plant growth in a controlled greenhouse setting.
  • Observational Study: Monitoring wildlife populations in a natural habitat to assess the effects of climate change on species distribution.

How QuestionPro Research Can Help in Experimental vs Observational Studies

Choosing between experimental and observational studies is a critical decision that can significantly impact the outcomes and interpretations of a study. QuestionPro Research offers powerful tools and features that can enhance both types of studies, giving researchers the flexibility and capability to gather, analyze, and interpret data effectively.

Enhancing Experimental Studies with QuestionPro

Experimental studies require a high degree of control over variables, randomization, and, often, repeated trials to establish causal relationships. QuestionPro excels in facilitating these requirements through several key features:

  • Survey Design and Distribution: With QuestionPro, researchers can design intricate surveys tailored to their experimental needs. The platform supports random assignment of participants to different groups, ensuring unbiased distribution and enhancing the study’s validity.
  • Data Collection and Management: Real-time data collection and management tools allow researchers to monitor responses as they come in. This is crucial for experimental studies where data collection timing and sequence can impact the results.
  • Advanced Analytics: QuestionPro offers robust analytical tools that can handle complex data sets, enabling researchers to conduct in-depth statistical analyses to determine the effects of the experimental interventions.

Supporting Observational Studies with QuestionPro

Observational studies involve gathering data without manipulating variables, focusing on natural settings and real-world scenarios. QuestionPro’s capabilities are well-suited for these studies as well:

  • Customizable Surveys: Researchers can create detailed surveys to capture a wide range of observational data. QuestionPro’s customizable templates and question types allow for flexibility in capturing nuanced information.
  • Mobile Data Collection: For field research, QuestionPro’s mobile app enables data collection on the go, making it easier to conduct studies in diverse settings without internet connectivity.
  • Longitudinal Data Tracking: Observational studies often require data collection over extended periods. QuestionPro’s platform supports longitudinal studies, allowing researchers to track changes and trends.

Experimental and observational studies are essential tools in the researcher’s toolkit. Each serves a unique purpose and offers distinct advantages and limitations. By understanding their differences, researchers can choose the most appropriate study design for their specific objectives, ensuring their findings are valid and applicable to real-world situations.

Whether establishing causality through experimental studies or exploring correlations with observational research designs, the insights gained from these methodologies continue to shape our understanding of the world around us. 

Whether conducting experimental or observational studies, QuestionPro Research provides a comprehensive suite of tools that enhance research efficiency, accuracy, and depth. By leveraging its advanced features, researchers can ensure that their studies are well-designed, their data is robustly analyzed, and their conclusions are reliable and impactful.

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Control Group vs Experimental Group

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BA (Hons) Psychology, Princeton University

Julia Simkus is a graduate of Princeton University with a Bachelor of Arts in Psychology. She is currently studying for a Master's Degree in Counseling for Mental Health and Wellness in September 2023. Julia's research has been published in peer reviewed journals.

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In a controlled experiment , scientists compare a control group, and an experimental group is identical in all respects except for one difference – experimental manipulation.

Differences

Unlike the experimental group, the control group is not exposed to the independent variable under investigation. So, it provides a baseline against which any changes in the experimental group can be compared.

Since experimental manipulation is the only difference between the experimental and control groups, we can be sure that any differences between the two are due to experimental manipulation rather than chance.

Almost all experimental studies are designed to include a control group and one or more experimental groups. In most cases, participants are randomly assigned to either a control or experimental group.

Because participants are randomly assigned to either group, we can assume that the groups are identical except for manipulating the independent variable in the experimental group.

It is important that every aspect of the experimental environment is the same and that the experimenters carry out the exact same procedures with both groups so researchers can confidently conclude that any differences between groups are actually due to the difference in treatments.

Control Group

A control group consists of participants who do not receive any experimental treatment. The control participants serve as a comparison group.

The control group is matched as closely as possible to the experimental group, including age, gender, social class, ethnicity, etc.

The difference between the control and experimental groups is that the control group is not exposed to the independent variable , which is thought to be the cause of the behavior being investigated.

Researchers will compare the individuals in the control group to those in the experimental group to isolate the independent variable and examine its impact.

The control group is important because it serves as a baseline, enabling researchers to see what impact changes to the independent variable produce and strengthening researchers’ ability to draw conclusions from a study.

Without the presence of a control group, a researcher cannot determine whether a particular treatment truly has an effect on an experimental group.

Control groups are critical to the scientific method as they help ensure the internal validity of a study.

Assume you want to test a new medication for ADHD . One group would receive the new medication, and the other group would receive a pill that looked exactly the same as the one that the others received, but it would be a placebo. The group that takes the placebo would be the control group.

Types of Control Groups

Positive control group.

  • A positive control group is an experimental control that will produce a known response or the desired effect.
  • A positive control is used to ensure a test’s success and confirm an experiment’s validity.
  • For example, when testing for a new medication, an already commercially available medication could serve as the positive control.

Negative Control Group

  • A negative control group is an experimental control that does not result in the desired outcome of the experiment.
  • A negative control is used to ensure that there is no response to the treatment and help identify the influence of external factors on the test.
  • An example of a negative control would be using a placebo when testing for a new medication.

Experimental Group

An experimental group consists of participants exposed to a particular manipulation of the independent variable. These are the participants who receive the treatment of interest.

Researchers will compare the responses of the experimental group to those of a control group to see if the independent variable impacted the participants.

An experiment must have at least one control group and one experimental group; however, a single experiment can include multiple experimental groups, which are all compared against the control group.

Having multiple experimental groups enables researchers to vary different levels of an experimental variable and compare the effects of these changes to the control group and among each other.

Assume you want to study to determine if listening to different types of music can help with focus while studying.

You randomly assign participants to one of three groups: one group that listens to music with lyrics, one group that listens to music without lyrics, and another group that listens to no music.

The group of participants listening to no music while studying is the control group, and the groups listening to music, whether with or without lyrics, are the two experimental groups.

Frequently Asked Questions

1. what is the difference between the control group and the experimental group in an experimental study.

Put simply; an experimental group is a group that receives the variable, or treatment, that the researchers are testing, whereas the control group does not. These two groups should be identical in all other aspects.

2. What is the purpose of a control group in an experiment

A control group is essential in experimental research because it:

Provides a baseline against which the effects of the manipulated variable (the independent variable) can be measured.

Helps to ensure that any changes observed in the experimental group are indeed due to the manipulation of the independent variable and not due to other extraneous or confounding factors.

Helps to account for the placebo effect, where participants’ beliefs about the treatment can influence their behavior or responses.

In essence, it increases the internal validity of the results and the confidence we can have in the conclusions.

3. Do experimental studies always need a control group?

Not all experiments require a control group, but a true “controlled experiment” does require at least one control group. For example, experiments that use a within-subjects design do not have a control group.

In  within-subjects designs , all participants experience every condition and are tested before and after being exposed to treatment.

These experimental designs tend to have weaker internal validity as it is more difficult for a researcher to be confident that the outcome was caused by the experimental treatment and not by a confounding variable.

4. Can a study include more than one control group?

Yes, studies can include multiple control groups. For example, if several distinct groups of subjects do not receive the treatment, these would be the control groups.

5. How is the control group treated differently from the experimental groups?

The control group and the experimental group(s) are treated identically except for one key difference: exposure to the independent variable, which is the factor being tested. The experimental group is subjected to the independent variable, whereas the control group is not.

This distinction allows researchers to measure the effect of the independent variable on the experimental group by comparing it to the control group, which serves as a baseline or standard.

Bailey, R. A. (2008). Design of Comparative Experiments. Cambridge University Press. ISBN 978-0-521-68357-9.

Hinkelmann, Klaus; Kempthorne, Oscar (2008). Design and Analysis of Experiments, Volume I: Introduction to Experimental Design (2nd ed.). Wiley. ISBN 978-0-471-72756-9.

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In experimental design, a group of research participants or subjects exposed to an independent variable in order to examine the causal effect of that treatment on a dependent variable. Compare control group.

From:   experimental group   in  A Dictionary of Psychology »

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Control Group Definition and Examples

Control Group in an Experiment

The control group is the set of subjects that does not receive the treatment in a study. In other words, it is the group where the independent variable is held constant. This is important because the control group is a baseline for measuring the effects of a treatment in an experiment or study. A controlled experiment is one which includes one or more control groups.

  • The experimental group experiences a treatment or change in the independent variable. In contrast, the independent variable is constant in the control group.
  • A control group is important because it allows meaningful comparison. The researcher compares the experimental group to it to assess whether or not there is a relationship between the independent and dependent variable and the magnitude of the effect.
  • There are different types of control groups. A controlled experiment has one more control group.

Control Group vs Experimental Group

The only difference between the control group and experimental group is that subjects in the experimental group receive the treatment being studied, while participants in the control group do not. Otherwise, all other variables between the two groups are the same.

Control Group vs Control Variable

A control group is not the same thing as a control variable. A control variable or controlled variable is any factor that is held constant during an experiment. Examples of common control variables include temperature, duration, and sample size. The control variables are the same for both the control and experimental groups.

Types of Control Groups

There are different types of control groups:

  • Placebo group : A placebo group receives a placebo , which is a fake treatment that resembles the treatment in every respect except for the active ingredient. Both the placebo and treatment may contain inactive ingredients that produce side effects. Without a placebo group, these effects might be attributed to the treatment.
  • Positive control group : A positive control group has conditions that guarantee a positive test result. The positive control group demonstrates an experiment is capable of producing a positive result. Positive controls help researchers identify problems with an experiment.
  • Negative control group : A negative control group consists of subjects that are not exposed to a treatment. For example, in an experiment looking at the effect of fertilizer on plant growth, the negative control group receives no fertilizer.
  • Natural control group : A natural control group usually is a set of subjects who naturally differ from the experimental group. For example, if you compare the effects of a treatment on women who have had children, the natural control group includes women who have not had children. Non-smokers are a natural control group in comparison to smokers.
  • Randomized control group : The subjects in a randomized control group are randomly selected from a larger pool of subjects. Often, subjects are randomly assigned to either the control or experimental group. Randomization reduces bias in an experiment. There are different methods of randomly assigning test subjects.

Control Group Examples

Here are some examples of different control groups in action:

Negative Control and Placebo Group

For example, consider a study of a new cancer drug. The experimental group receives the drug. The placebo group receives a placebo, which contains the same ingredients as the drug formulation, minus the active ingredient. The negative control group receives no treatment. The reason for including the negative group is because the placebo group experiences some level of placebo effect, which is a response to experiencing some form of false treatment.

Positive and Negative Controls

For example, consider an experiment looking at whether a new drug kills bacteria. The experimental group exposes bacterial cultures to the drug. If the group survives, the drug is ineffective. If the group dies, the drug is effective.

The positive control group has a culture of bacteria that carry a drug resistance gene. If the bacteria survive drug exposure (as intended), then it shows the growth medium and conditions allow bacterial growth. If the positive control group dies, it indicates a problem with the experimental conditions. A negative control group of bacteria lacking drug resistance should die. If the negative control group survives, something is wrong with the experimental conditions.

  • Bailey, R. A. (2008).  Design of Comparative Experiments . Cambridge University Press. ISBN 978-0-521-68357-9.
  • Chaplin, S. (2006). “The placebo response: an important part of treatment”.  Prescriber . 17 (5): 16–22. doi: 10.1002/psb.344
  • Hinkelmann, Klaus; Kempthorne, Oscar (2008).  Design and Analysis of Experiments, Volume I: Introduction to Experimental Design  (2nd ed.). Wiley. ISBN 978-0-471-72756-9.
  • Pithon, M.M. (2013). “Importance of the control group in scientific research.” Dental Press J Orthod . 18 (6):13-14. doi: 10.1590/s2176-94512013000600003
  • Stigler, Stephen M. (1992). “A Historical View of Statistical Concepts in Psychology and Educational Research”. American Journal of Education . 101 (1): 60–70. doi: 10.1086/444032

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Biology Dictionary

Controlled Experiment

BD Editors

Reviewed by: BD Editors

Controlled Experiment Definition

A controlled experiment is a scientific test that is directly manipulated by a scientist, in order to test a single variable at a time. The variable being tested is the independent variable , and is adjusted to see the effects on the system being studied. The controlled variables are held constant to minimize or stabilize their effects on the subject. In biology, a controlled experiment often includes restricting the environment of the organism being studied. This is necessary to minimize the random effects of the environment and the many variables that exist in the wild.

In a controlled experiment, the study population is often divided into two groups. One group receives a change in a certain variable, while the other group receives a standard environment and conditions. This group is referred to as the control group , and allows for comparison with the other group, known as the experimental group . Many types of controls exist in various experiments, which are designed to ensure that the experiment worked, and to have a basis for comparison. In science, results are only accepted if it can be shown that they are statistically significant . Statisticians can use the difference between the control group and experimental group and the expected difference to determine if the experiment supports the hypothesis , or if the data was simply created by chance.

Examples of Controlled Experiment

Music preference in dogs.

Do dogs have a taste in music? You might have considered this, and science has too. Believe it or not, researchers have actually tested dog’s reactions to various music genres. To set up a controlled experiment like this, scientists had to consider the many variables that affect each dog during testing. The environment the dog is in when listening to music, the volume of the music, the presence of humans, and even the temperature were all variables that the researches had to consider.

In this case, the genre of the music was the independent variable. In other words, to see if dog’s change their behavior in response to different kinds of music, a controlled experiment had to limit the interaction of the other variables on the dogs. Usually, an experiment like this is carried out in the same location, with the same lighting, furniture, and conditions every time. This ensures that the dogs are not changing their behavior in response to the room. To make sure the dogs don’t react to humans or simply the noise of the music, no one else can be in the room and the music must be played at the same volume for each genre. Scientist will develop protocols for their experiment, which will ensure that many other variables are controlled.

This experiment could also split the dogs into two groups, only testing music on one group. The control group would be used to set a baseline behavior, and see how dogs behaved without music. The other group could then be observed and the differences in the group’s behavior could be analyzed. By rating behaviors on a quantitative scale, statistics can be used to analyze the difference in behavior, and see if it was large enough to be considered significant. This basic experiment was carried out on a large number of dogs, analyzing their behavior with a variety of different music genres. It was found that dogs do show more relaxed and calm behaviors when a specific type of music plays. Come to find out, dogs enjoy reggae the most.

Scurvy in Sailors

In the early 1700s, the world was a rapidly expanding place. Ships were being built and sent all over the world, carrying thousands and thousands of sailors. These sailors were mostly fed the cheapest diets possible, not only because it decreased the costs of goods, but also because fresh food is very hard to keep at sea. Today, we understand that lack of essential vitamins and nutrients can lead to severe deficiencies that manifest as disease. One of these diseases is scurvy.

Scurvy is caused by a simple vitamin C deficiency, but the effects can be brutal. Although early symptoms just include general feeling of weakness, the continued lack of vitamin C will lead to a breakdown of the blood cells and vessels that carry the blood. This results in blood leaking from the vessels. Eventually, people bleed to death internally and die. Before controlled experiments were commonplace, a simple physician decided to tackle the problem of scurvy. James Lind, of the Royal Navy, came up with a simple controlled experiment to find the best cure for scurvy.

He separated sailors with scurvy into various groups. He subjected them to the same controlled condition and gave them the same diet, except one item. Each group was subjected to a different treatment or remedy, taken with their food. Some of these remedies included barley water, cider and a regiment of oranges and lemons. This created the first clinical trial , or test of the effectiveness of certain treatments in a controlled experiment. Lind found that the oranges and lemons helped the sailors recover fast, and within a few years the Royal Navy had developed protocols for growing small leafy greens that contained high amounts of vitamin C to feed their sailors.

Related Biology Terms

  • Field Experiment – An experiment conducted in nature, outside the bounds of total control.
  • Independent Variable – The thing in an experiment being changed or manipulated by the experimenter to see effects on the subject.
  • Controlled Variable – A thing that is normalized or standardized across an experiment, to remove it from having an effect on the subject being studied.
  • Control Group – A group of subjects in an experiment that receive no independent variable, or a normalized amount, to provide comparison.

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  • Introduction

Comparison with controlled study design

Natural experiments as quasi experiments, instrumental variables.

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natural experiment , observational study in which an event or a situation that allows for the random or seemingly random assignment of study subjects to different groups is exploited to answer a particular question. Natural experiments are often used to study situations in which controlled experimentation is not possible, such as when an exposure of interest cannot be practically or ethically assigned to research subjects. Situations that may create appropriate circumstances for a natural experiment include policy changes, weather events, and natural disasters. Natural experiments are used most commonly in the fields of epidemiology , political science , psychology , and social science .

Key features of experimental study design include manipulation and control. Manipulation, in this context , means that the experimenter can control which research subjects receive which exposures. For instance, subjects randomized to the treatment arm of an experiment typically receive treatment with the drug or therapy that is the focus of the experiment, while those in the control group receive no treatment or a different treatment. Control is most readily accomplished through random assignment, which means that the procedures by which participants are assigned to a treatment and control condition ensure that each has equal probability of assignment to either group. Random assignment ensures that individual characteristics or experiences that might confound the treatment results are, on average, evenly distributed between the two groups. In this way, at least one variable can be manipulated, and units are randomly assigned to the different levels or categories of the manipulated variables.

In epidemiology, the gold standard in research design generally is considered to be the randomized control trial (RCT). RCTs, however, can answer only certain types of epidemiologic questions, and they are not useful in the investigation of questions for which random assignment is either impracticable or unethical. The bulk of epidemiologic research relies on observational data, which raises issues in drawing causal inferences from the results. A core assumption for drawing causal inference is that the average outcome of the group exposed to one treatment regimen represents the average outcome the other group would have had if they had been exposed to the same treatment regimen. If treatment is not randomly assigned, as in the case of observational studies, the assumption that the two groups are exchangeable (on both known and unknown confounders) cannot be assumed to be true.

As an example, suppose that an investigator is interested in the effect of poor housing on health. Because it is neither practical nor ethical to randomize people to variable housing conditions, this subject is difficult to study using an experimental approach. However, if a housing policy change, such as a lottery for subsidized mortgages, was enacted that enabled some people to move to more desirable housing while leaving other similar people in their previous substandard housing, it might be possible to use that policy change to study the effect of housing change on health outcomes. In another example, a well-known natural experiment in Helena , Montana, smoking was banned from all public places for a six-month period. Investigators later reported a 60-percent drop in heart attacks for the study area during the time the ban was in effect.

Because natural experiments do not randomize participants into exposure groups, the assumptions and analytical techniques customarily applied to experimental designs are not valid for them. Rather, natural experiments are quasi experiments and must be thought about and analyzed as such. The lack of random assignment means multiple threats to causal inference , including attrition , history, testing, regression , instrumentation, and maturation, may influence observed study outcomes. For this reason, natural experiments will never unequivocally determine causation in a given situation. Nevertheless, they are a useful method for researchers, and if used with care they can provide additional data that may help with a research question and that may not be obtainable in any other way.

The major limitation in inferring causation from natural experiments is the presence of unmeasured confounding. One class of methods designed to control confounding and measurement error is based on instrumental variables (IV). While useful in a variety of applications, the validity and interpretation of IV estimates depend on strong assumptions, the plausibility of which must be considered with regard to the causal relation in question.

experimental group definition environmental science

In particular, IV analyses depend on the assumption that subjects were effectively randomized, even if the randomization was accidental (in the case of an administrative policy change or exposure to a natural disaster) and adherence to random assignment was low. IV methods can be used to control for confounding in observational studies, to control for confounding due to noncompliance, and to correct for misclassification.

IV analysis, however, can produce serious biases in effect estimates. It can also be difficult to identify the particular subpopulation to which the causal effect IV estimate applies. Moreover, IV analysis can add considerable imprecision to causal effect estimates. Small sample size poses an additional challenge in applying IV methods.

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Control group

In general biology i.

A control group is a group in an experiment that does not receive the treatment being tested, serving as a baseline to compare the effects of the treatment. It helps researchers determine if changes in the experimental group are due to the treatment or other factors.

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  • General Biology I - 1.1 The Science of Biology

5 Must Know Facts For Your Next Test

  • The control group is essential for establishing causality in an experiment.
  • It should be as similar as possible to the experimental group except for the factor being tested.
  • Control groups help eliminate alternative explanations by providing a comparison point.
  • Sometimes placebos are used in control groups, especially in medical studies.
  • Data from control groups can reveal natural variations and external influences on experimental results.

Review Questions

  • Why is a control group important in an experiment?
  • How does a control group differ from an experimental group?
  • What role do placebos play in control groups?

Related terms

Experimental Group : one sentence definition

Placebo : one sentence definition

Variable : one sentence definition

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The state-of-the-art of mycobacterium chimaera infections and the causal link with health settings: a systematic review.

experimental group definition environmental science

1. Introduction

2. materials and methods, 4. discussion, 4.1. mycobacterium chimaera’s characteristics and ecosystem, 4.2. heater-cooler units, medical devices, water, and air-conditioned implants, 4.3. incubation period and symptoms presentation, 4.4. presence in the lung system, 4.5. modality of transmission, 4.6. detection, 4.7. disinfection, 4.8. causal link assessment, 5. limitations, 6. conclusions, supplementary materials, author contributions, institutional review board statement, informed consent statement, data availability statement, conflicts of interest, abbreviations.

MAC mycobacterium avium complex
NTM non-tuberculosis mycobacterium
M. chimaeraMycobacterium chimaera
HCU heater-cooler units
OPPP opportunistic premise plumbing pathogens
ECMO extra-corporal mechanical oxygenation
HAI healthcare-associated infection
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ReferencesAuthor, YearN. of Patients SurgeryMean Time of Presentation If Previous SurgerySetting (Country)Organ and/or Tissue Involved
[ ](Bills et al., 2009)1NoneNaNot healthcare (USA)Lung, nodules in chronic obstructive pulmonary disease
[ ](Cohen-Bacrie et al., 2011)1NoneNaPossible frequent healthcare contact (Réunion Island, FR)Lung infections in cystic fibrosis
[ ](Alhanna et al., 2012)1NoneNaNot healthcare (Germany)Lung infection
[ ](Gunaydin et al., 2013)5 (of 90)NoneNaPossible healthcare contact (Turkey)Lung (reassessment of sputum specimens)
[ ](Boyle et al., 2015)125 (of 448)NoneNaPossible healthcare contact (USA)Lung (reassessment of sputum specimens)
[ ](Mwikuma et al., 2015)
1 (of 54) NoneNaNot healthcare (Zambia)Lung (reassessment of sputum specimens)
[ ](Moon et al., 2016)11NoneNaNot healthcare (South Korea)Lung infection (reassessment of sputum specimens)
[ ](Moutsoglou et al., 2017)1NoneNaNot healthcare (USA)Disseminated with spinal osteomyelitis and discitis
[ ](Bursle et al., 2017)1Tricuspid valve repair and mitral annuloplasty13 monthsUnderwent surgery (Australia)Disseminated
[ ]Kim et al., 20178 (of 91)NoneNaPossible healthcare contact (Korea)Lung (reassessment of sputum specimens)
[ ](Chand et al., 2017) *4Valvular cardiac surgery 1.15 (0.25–5.1) yearsUnderwent surgery (UK)1 osteomyelitis and 3 disseminated
[ ](Truden et al., 2018)49 (of 102)NoneNaPossible healthcare contact (Slovenia)Lung (reassessment of sputum specimens)
[ ](Larcher et al., 2019) 4NoneNaPossible frequent healthcare contact (France)Lung (reassessment of sputum specimens in cystic fibrosis)
[ ](Shafizadeh et al., 2019) *5Valvular cardiac surgery20.6 (14–29) monthsUnderwent surgery (USA)Disseminated with liver infection
[ ](Rosero and Shams, 2019)1None but operating room nurse 10 years ago>10 yearsPossible frequent healthcare contact (USA)Lung infection
[ ](Watanabe et al., 2020)1NoneNaNot healthcare (Japan)Tendons, hand tenosynovitis
[ ](Chen et al., 2020)28NoneNaNot healthcare (Taiwan)Lung infection (reassessment of sputum specimens)
[ ](Maalouly et al., 2020)1Kidney transplantationOne weekUnderwent surgery (Belgium)Kidney, urinary tract infection in a kidney transplant recipient with concomitant Mycobacterium malmoense lung infection and fibro anthracosis
[ ](de Melo Carvalho et al., 2020)1NoneNaPossible healthcare contact (Portugal)Disseminated in B-cell lymphoma
[ ](Sharma et al., 2020)2NoneNaNot healthcare (India)Meninges, meningitis
[ ](Zabost et al., 2021)88 (of 200)NoneNaPossible healthcare contact (Poland)Lung infection (reassessment of sputum specimens)
[ ](Kim et al., 2021)4 (of 320) NoneNaPossible healthcare contact (Korea) Lung infection (reassessment of sputum specimens)
[ ](Kavvalou et al., 2022)1NoneNaPossible healthcare contact (Germany)Central venous catheter infection in cystic fibrosis
[ ](Robinson et al., 2022)1NoneNaNot healthcare (USA)Lung infection in drug abuser
[ ](Ahmad et al., 2022)1NoneNaNot healthcare (USA)Lung infection in sarcoidosis
[ ](George et al., 2022)1NoneNaNot healthcare (India)Skin, periapical abscess with chin ulcer
[ ](Lin et al., 2022)1NoneNaPossible frequent healthcare contact (Taiwan)Disseminated in adult-onset immunodeficiency syndrome
[ ](Łyżwa et al., 2022)1NoneNaNot healthcare (Poland)Lung infection in silicosis
[ ](McLaughlin et al., 2022)1Coronary artery bypass grafting1 yearUnderwent surgery (USA)Tendons, hand tenosynovitis in ipsilateral elbow wound in fisherman
[ ](Gross et al., 2023)23NoneNaHealthcare (USA)Lung infections in cystic fibrosis (genomic analysis for cluster correlation to hospital outbreaks)
[ ](Azzarà et al., 2023)1NoneNaPossible healthcare contact (Italy)Lung infection in lung adenocarcinoma treated with immune checkpoint inhibitors
[ ](Pradhan et al., 2023)1Bioprosthetic mitral valve replacement7 yearsUnderwent surgery (Australia)Spinal osteomyelitis and discitis
[ ](Garcia-Prieto et al., 2024)1NoneNaNot healthcare (Spain)Lung infection in fibro anthracosis
[ ](Paul et al., 2024)1NoneNaPossible healthcare contact (UK)Lung infection in unilateral pulmonary artery agenesis on the right side
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Bolcato, V.; Bassetti, M.; Basile, G.; Bianco Prevot, L.; Speziale, G.; Tremoli, E.; Maffessanti, F.; Tronconi, L.P. The State-of-the-Art of Mycobacterium chimaera Infections and the Causal Link with Health Settings: A Systematic Review. Healthcare 2024 , 12 , 1788. https://doi.org/10.3390/healthcare12171788

Bolcato V, Bassetti M, Basile G, Bianco Prevot L, Speziale G, Tremoli E, Maffessanti F, Tronconi LP. The State-of-the-Art of Mycobacterium chimaera Infections and the Causal Link with Health Settings: A Systematic Review. Healthcare . 2024; 12(17):1788. https://doi.org/10.3390/healthcare12171788

Bolcato, Vittorio, Matteo Bassetti, Giuseppe Basile, Luca Bianco Prevot, Giuseppe Speziale, Elena Tremoli, Francesco Maffessanti, and Livio Pietro Tronconi. 2024. "The State-of-the-Art of Mycobacterium chimaera Infections and the Causal Link with Health Settings: A Systematic Review" Healthcare 12, no. 17: 1788. https://doi.org/10.3390/healthcare12171788

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COMMENTS

  1. Experimental Group

    Experimental Group Definition. In a comparative experiment, the experimental group (aka the treatment group) is the group being tested for a reaction to a change in the variable. There may be experimental groups in a study, each testing a different level or amount of the variable. The other type of group, the control group, can show the effects ...

  2. Understanding Experimental Groups

    Experimental Group Definition. An experimental group in a scientific experiment is the group on which the experimental procedure is performed. The independent variable is changed for the group and the response or change in the dependent variable is recorded. In contrast, the group that does not receive the treatment or in which the independent ...

  3. Experimental Group

    The experimental group, in scientific research, refers to the group subjected to specific changes or treatments in a variable to observe and evaluate ... The rationale behind this is the intricate interplay between genetics and environmental factors. When organisms with distinct genetic backgrounds are exposed to a consistent variable, the ...

  4. The Difference Between Control Group and Experimental Group

    The control group and experimental group are compared against each other in an experiment. The only difference between the two groups is that the independent variable is changed in the experimental group. The independent variable is "controlled", or held constant, in the control group. A single experiment may include multiple experimental ...

  5. Experimental & Control Group

    Experimental and control groups are the two main groups found in an experiment, each serving a slightly different purpose. Experimental groups are being manipulated to try and change the out come ...

  6. Experimental Group

    Conclusion. Experimental treatment studies function in the way that they involve different groups, one of which serves as a control group to provide a baseline for the estimation of the treatment effect. The treatment therefore defines the group as independent variable, which is manipulated and therefore makes the investigation an experiment.

  7. Experimental & Control Group

    An experimental group is the group in an experiment that receives the variable being tested. One variable is tested at a time. The experimental group is compared to a control group, which does not ...

  8. Khan Academy

    If you're seeing this message, it means we're having trouble loading external resources on our website. If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

  9. Experimental Group

    As noted in the Experimental Designs entry, experimental designs are those in which the influence(s) of interest are controlled by the research scientist. An experimental group is a group of subjects who receive a particular treatment or intervention. Experimental subjects are randomly assigned to one of the treatment groups so that many potential influences that cannot be controlled for (e.g ...

  10. Environmental science

    environmental science, interdisciplinary academic field that draws on ecology, geology, meteorology, biology, chemistry, engineering, and physics to study environmental problems and human impacts on the environment. Environmental science is a quantitative discipline with both applied and theoretical aspects and has been influential in informing ...

  11. Experimental Group

    Definition. The experimental group refers to the group in an experiment that receives the treatment or intervention being studied. Related terms. Control Group: The control group is the group in an experiment that does not receive any treatment or intervention, serving as a baseline for comparison.

  12. Experimental vs Observational Studies: Differences & Examples

    Environmental Science. Experimental Study: Testing the impact of a specific pollutant on plant growth in a controlled greenhouse setting. Observational Study: Monitoring wildlife populations in a natural habitat to assess the effects of climate change on species distribution. How QuestionPro Research Can Help in Experimental vs Observational ...

  13. PDF Design of Experiments in Ecological and Environmental Problems: methods

    Steps to Perform DOE. Set experimental objectives. Select process variables. Select an experimental design. Execute the experimental design. Check that data are consistent with experimental design assumptions. Analyze and interpret results. Conclude/Restart the loop.

  14. Experimental variables

    The independent variable is the specific experimental variable that is deliberately changed by the scientist. The dependent variable is the outcome or result that is measured and observed in response to changes in the independent variable. A control group is a group in an experiment that does not receive any treatment or manipulation and serves ...

  15. Control Group Vs Experimental Group In Science

    Put simply; an experimental group is a group that receives the variable, or treatment, that the researchers are testing, whereas the control group does not. These two groups should be identical in all other aspects. 2. What is the purpose of a control group in an experiment.

  16. Control group

    control group, the standard to which comparisons are made in an experiment. Many experiments are designed to include a control group and one or more experimental groups; in fact, some scholars reserve the term experiment for study designs that include a control group. Ideally, the control group and the experimental groups are identical in every ...

  17. Experimental group

    Quick Reference. In experimental design, a group of research participants or subjects exposed to an independent variable in order to examine the causal effect of that treatment on a dependent variable. Compare control group. From: experimental group in A Dictionary of Psychology ». Subjects: Medicine and health — Clinical Medicine.

  18. Control Group Definition and Examples

    A control group is not the same thing as a control variable. A control variable or controlled variable is any factor that is held constant during an experiment. Examples of common control variables include temperature, duration, and sample size. The control variables are the same for both the control and experimental groups.

  19. Controlled Experiment

    Controlled Experiment Definition. A controlled experiment is a scientific test that is directly manipulated by a scientist, in order to test a single variable at a time. The variable being tested is the independent variable, and is adjusted to see the effects on the system being studied. The controlled variables are held constant to minimize or ...

  20. Create a culture of experiments in environmental programs

    Formal experimentation in environmental programs is absent because science typically stops when implementation starts. Over the past five decades, governmental and nongovernmental actors have invested substantial resources to understand the status and trends of myriad environmental indicators. These investments have been motivated by scientific ...

  21. Controlled Experiment

    Environmental Science ... A controlled experiment is defined as an experiment in which all the variable factors in an experimental group and a comparison control group are kept the same except for ...

  22. Natural experiment

    natural experiment, observational study in which an event or a situation that allows for the random or seemingly random assignment of study subjects to different groups is exploited to answer a particular question. Natural experiments are often used to study situations in which controlled experimentation is not possible, such as when an ...

  23. Control group

    Definition. A control group is a group in an experiment that does not receive the treatment being tested, serving as a baseline to compare the effects of the treatment. It helps researchers determine if changes in the experimental group are due to the treatment or other factors.

  24. 5: Experimental Design

    Experimental design is a discipline within statistics concerned with the analysis and design of experiments. Design is intended to help research create experiments such that cause and effect can be established from tests of the hypothesis. We introduced elements of experimental design in Chapter 2.4. Here, we expand our discussion of ...

  25. The State-of-the-Art of Mycobacterium chimaera Infections and the

    (1) Background. A definition of healthcare-associated infections is essential also for the attribution of the restorative burden to healthcare facilities in case of harm and for clinical risk management strategies. Regarding M. chimaera infections, there remains several issues on the ecosystem and pathogenesis. We aim to review the scientific evidence on M. chimaera beyond cardiac surgery, and ...