chapter 12 experiments and observational studies answers

Chapter 12, Lesson 1: Experiments, Surveys, and Observational Studies

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Stats Modeling the World 4th

David e. bock, paul f. velleman, richard d. de veaux, experiments and observational studies - all with video answers.

Chapter Questions

For his Statistics class experiment, researcher J. Gilbert decided to study how parents' income affects children's performance on standardized tests like the SAT. He proposed to collect information from a random sample of test takers and examine the relationship between parental income and SAT score. a) Is this an experiment? If not, what kind of study is it? b) If there is relationship between parental income and SAT score, why can't we conclude that differences in score are caused by differences in parental income?

Researchers who examined health records of thousands of males found that men who died of myocardial infarction (heart attack) tended to be shorter than men who did not. a) Is this an experiment? If not, what kind of study is it? b) Is it correct to conclude that shorter men are at higher risk for heart attack? Explain.

Multiple sclerosis (MS) is an autoimmune disease that strikes more often the farther people live from the equator. Could vitamin D - which most people get from the sun's ultraviolet rays - be a factor? Researchers compared vitamin D levels in blood samples from 150 U.S. military personnel who have developed MS with blood samples of nearly 300 who have not. The samples were taken, on average, five years before the disease was diagnosed. Those with the highest blood vitamin D levels had a $62 \%$ lower risk of MS than those with the lowest levels. (The link was only in whites, not in blacks or Hispanics.) a) What kind of study was this? b) Is that an appropriate choice for investigating this problem? Explain. c) Who were the subjects? d) What were the variables?

When spending large amounts to purchase advertising time, companies want to know what audience they'll reach. In January $2011,$ a poll by The Hollywood Reporter asked randomly selected American adults whether they planned to watch the upcoming Super Bowl. Men and women were asked separately whether they were looking forward more to the football game or to watching the commercials. Among the men, who planned on watching, $70 \%$ were watching for the game. Among women, $60 \%$ were looking forward primarily to the game. a) Was this a stratified sample or a blocked experiment? Explain. b) Was the design of the study appropriate for the advertisers' questions?

People generally don't think "health" when they hear the word "maggot," but one experiment tested the ability of maggots to remove dead tissue from open wounds that would not heal on their own. Sterile maggots were placed in a small pouch which, in turn, was placed on the wound. 100 men with wounds on their lower limbs were randomly assigned to receive either a traditional surgical treatment or maggot therapy. After eight days, the percentage of dead tissue in the wounds that underwent maggot treatment was $54.5 \%,$ compared to $66.5 \%$ with the surgical treatment. (The difference decreased with time, and the advantage disappeared by about day $15 .$ ) Neither patients nor the doctors evaluating the wounds knew which therapy had been applied. (Patients were blindfolded as bandages were changed.) Surprisingly, the number of patients that reported a crawling sensation in their wound was about the same in both groups (http://www.myhealthnewsdaily.com/2030maggots-clean-wounds-faster-surgeons.html) a) What kind of study was this? b) Is that an appropriate choice for this investigation? c) Who were the subjects? d) Identify the treatment and response variables.

Coffee stations in offices often just ask users to leave money in a tray to pay for their coffee, but many people cheat. Researchers at Newcastle University replaced the picture of flowers on the wall behind the coffee station with a picture of staring eyes. They found that the average contribution increased significantly above the well-established standard when people felt they were being watched, even though the eyes were patently not real. (NY Times $12 / 10 / 06$ ) a) Was this a survey, an observational study, or an experiment? How can we tell? b) Identify the variables. c) What does "increased significantly" mean in a statistical sense?

Read each brief report of statistical research, and identify a) whether it was an observational study or an experiment. If it was an observational study, identify (if possible) b) whether it was retrospective or prospective. c) the subjects studied and how they were selected. d) the parameter of interest. e) the nature and scope of the conclusion the study can reach. If it was an experiment, identify (if possible) b) the subjects studied. c) the factor(s) in the experiment and the number of levels for each. d) the number of treatments. e) the response variable measured. f) the design (completely randomized, blocked, or matched). g) whether it was blind (or double-blind). h) the nature and scope of the conclusion the experiment can reach. Superglue 130 patients with eligible lacerations randomly assigned to have the wound closed either with Octylcyanoacrylate Tissue Adhesive (essentially superglue) or with traditional sutures. When evaluated at the end of the study, the two treatments worked equally well with regard to scarring, and the adhesive was less painful and worked faster.

Read each brief report of statistical research, and identify a) whether it was an observational study or an experiment. If it was an observational study, identify (if possible) b) whether it was retrospective or prospective. c) the subjects studied and how they were selected. d) the parameter of interest. e) the nature and scope of the conclusion the study can reach. If it was an experiment, identify (if possible) b) the subjects studied. c) the factor(s) in the experiment and the number of levels for each. d) the number of treatments. e) the response variable measured. f) the design (completely randomized, blocked, or matched). g) whether it was blind (or double-blind). h) the nature and scope of the conclusion the experiment can reach. Truancy A group of researchers analyzed three observational studies that followed children's attendance and mental health, among other things. They found that students who missed more school tended to have more incidences of depression. One study followed 20,745 secondary students in a random sample of all secondary schools in the United States. Another tracked 2,311 first graders at 18 Baltimore schools who were participating in an intervention program. The third study followed 671 students from first or fifth grade to their senior year in high risk areas of Eugene, OR, who had been randomly assigned to an intervention or to no intervention.

Read each brief report of statistical research, and identify a) whether it was an observational study or an experiment. If it was an observational study, identify (if possible) b) whether it was retrospective or prospective. c) the subjects studied and how they were selected. d) the parameter of interest. e) the nature and scope of the conclusion the study can reach. If it was an experiment, identify (if possible) b) the subjects studied. c) the factor(s) in the experiment and the number of levels for each. d) the number of treatments. e) the response variable measured. f) the design (completely randomized, blocked, or matched). g) whether it was blind (or double-blind). h) the nature and scope of the conclusion the experiment can reach. Hypertension In a test of roughly 200 older men and women, those with moderately high blood pressure (averaging $164 / 89 \mathrm{mm} \mathrm{Hg}$ ) did worse on tests of memory and reaction time than those with normal blood pressure. (Hypertension 36 [2000]: 1079)

Read each brief report of statistical research, and identify a) whether it was an observational study or an experiment. If it was an observational study, identify (if possible) b) whether it was retrospective or prospective. c) the subjects studied and how they were selected. d) the parameter of interest. e) the nature and scope of the conclusion the study can reach. If it was an experiment, identify (if possible) b) the subjects studied. c) the factor(s) in the experiment and the number of levels for each. d) the number of treatments. e) the response variable measured. f) the design (completely randomized, blocked, or matched). g) whether it was blind (or double-blind). h) the nature and scope of the conclusion the experiment can reach. Tossing and turning Is diet or exercise effective in combating insomnia? Some believe that cutting out desserts can help alleviate the problem, while others recommend exercise. Forty volunteers suffering from insomnia agreed to participate in a month-long test. Half were randomly assigned to a special no-desserts diet; the others continued desserts as usual. Half of the people in each of these groups were randomly assigned to an exercise program, while the others did not exercise. Those who ate no desserts and engaged in exercise showed the most improvement.

Read each brief report of statistical research, and identify a) whether it was an observational study or an experiment. If it was an observational study, identify (if possible) b) whether it was retrospective or prospective. c) the subjects studied and how they were selected. d) the parameter of interest. e) the nature and scope of the conclusion the study can reach. If it was an experiment, identify (if possible) b) the subjects studied. c) the factor(s) in the experiment and the number of levels for each. d) the number of treatments. e) the response variable measured. f) the design (completely randomized, blocked, or matched). g) whether it was blind (or double-blind). h) the nature and scope of the conclusion the experiment can reach. Alcohol and estrogen After menopause, some women take supplemental estrogen. There is some concern that if these women also drink alcohol, their estrogen levels will rise too high. Twelve volunteers who were receiving supplemental estrogen were randomly divided into two groups, as were 12 other volunteers not on estrogen. In each case, one group drank an alcoholic beverage, the other a nonalcoholic beverage. An hour later, everyone's estrogen level was checked. Only those on supplemental estrogen who drank alcohol showed a marked increase.

Read each brief report of statistical research, and identify a) whether it was an observational study or an experiment. If it was an observational study, identify (if possible) b) whether it was retrospective or prospective. c) the subjects studied and how they were selected. d) the parameter of interest. e) the nature and scope of the conclusion the study can reach. If it was an experiment, identify (if possible) b) the subjects studied. c) the factor(s) in the experiment and the number of levels for each. d) the number of treatments. e) the response variable measured. f) the design (completely randomized, blocked, or matched). g) whether it was blind (or double-blind). h) the nature and scope of the conclusion the experiment can reach. Dioxin Researchers have linked an increase in the incidence of breast cancer in Italy to dioxin released by an industrial accident in $1976 .$ The study identified 981 women who lived near the site of the accident and were under age 40 at the time. Fifteen of the women had developed breast cancer at an unusually young average age of 45 Medical records showed that they had heightened concentrations of dioxin in their blood and that each tenfold increase in dioxin level was associated with a doubling of the risk of breast cancer. (Science News, Aug. 3, 2002)

Read each brief report of statistical research, and identify a) whether it was an observational study or an experiment. If it was an observational study, identify (if possible) b) whether it was retrospective or prospective. c) the subjects studied and how they were selected. d) the parameter of interest. e) the nature and scope of the conclusion the study can reach. If it was an experiment, identify (if possible) b) the subjects studied. c) the factor(s) in the experiment and the number of levels for each. d) the number of treatments. e) the response variable measured. f) the design (completely randomized, blocked, or matched). g) whether it was blind (or double-blind). h) the nature and scope of the conclusion the experiment can reach. Boys and girls In 2002 the journal Science reported that a study of women in Finland indicated that having sons shortened the lifespans of mothers by about 34 weeks per son, but that daughters helped to lengthen the mothers' lives. The data came from church records from the period 1640 to 1870.

Read each brief report of statistical research, and identify a) whether it was an observational study or an experiment. If it was an observational study, identify (if possible) b) whether it was retrospective or prospective. c) the subjects studied and how they were selected. d) the parameter of interest. e) the nature and scope of the conclusion the study can reach. If it was an experiment, identify (if possible) b) the subjects studied. c) the factor(s) in the experiment and the number of levels for each. d) the number of treatments. e) the response variable measured. f) the design (completely randomized, blocked, or matched). g) whether it was blind (or double-blind). h) the nature and scope of the conclusion the experiment can reach. Herbal remedy Scientists at a major pharmaceutical firm investigated the effectiveness of an herbal compound to treat the common cold. They exposed each subject to a cold virus, then randomly assigned him or her either the herbal compound or a sugar solution known to have no effect on colds. Several days later they assessed the patient's condition, using a cold severity scale ranging from 0 to $5 .$ They found no evidence of benefits associated with the compound.

Read each brief report of statistical research, and identify a) whether it was an observational study or an experiment. If it was an observational study, identify (if possible) b) whether it was retrospective or prospective. c) the subjects studied and how they were selected. d) the parameter of interest. e) the nature and scope of the conclusion the study can reach. If it was an experiment, identify (if possible) b) the subjects studied. c) the factor(s) in the experiment and the number of levels for each. d) the number of treatments. e) the response variable measured. f) the design (completely randomized, blocked, or matched). g) whether it was blind (or double-blind). h) the nature and scope of the conclusion the experiment can reach. Depression The May $4,2000,$ issue of Science News reported that, contrary to popular belief, depressed individuals cry no more often in response to sad situations than non depressed people. Researchers studied 23 men and 48 women with major depression and 9 men and 24 women with no depression. They showed the subjects a sad film about a boy whose father has died, noting whether or not the subjects cried. Women cried more often than men, but there were no significant differences between the depressed and non depressed groups.

Read each brief report of statistical research, and identify a) whether it was an observational study or an experiment. If it was an observational study, identify (if possible) b) whether it was retrospective or prospective. c) the subjects studied and how they were selected. d) the parameter of interest. e) the nature and scope of the conclusion the study can reach. If it was an experiment, identify (if possible) b) the subjects studied. c) the factor(s) in the experiment and the number of levels for each. d) the number of treatments. e) the response variable measured. f) the design (completely randomized, blocked, or matched). g) whether it was blind (or double-blind). h) the nature and scope of the conclusion the experiment can reach. Vitamin C doping Some people who race greyhounds give the dogs large doses of vitamin $\mathrm{C}$ in the belief that the dogs will run faster. Investigators at the University of Florida tried three different diets in random order on each of five racing greyhounds. They were surprised to find that when the dogs ate high amounts of vitamin C they ran more slowly. (Science News, July 20,2002 )

Torn ACL Having at least one 15 -minute warm-up session per week resulted in a drastic reduction in tears in the anterior cruciate ligament (ACL). In a study involving about 4500 adolescent girls' soccer players in Sweden, one group was randomly assigned to warm up with a neuromuscular exercise session. This group had $64 \%$ fewer ACL tears than the control group. a) Is this an experiment or an observational study? Explain why. b) Identify the treatments in this study. What is the response variable? c) Give one statistical advantage of using only Swedish girls who played soccer in this study. d) Give one statistical disadvantage of using only Swedish girls who played soccer in this study.

Losing sleep An article entitled "TV Before Bed May Rob Teens of Sleep" reported on a study published online in Pediatrics in January of $2013 .$ The study found that students who watch TV before bedtime tend to go to sleep later than those who engaged in nonscreen sedentary activities before bed. Researchers contacted a nationally representative cross-sectional sample of teens in New Zealand, interviewing participants in person and following up with phone interviews, to look for a relationship between before-bed activities and the length of time before kids go to sleep. a) Is this an experiment? Explain why or why not. b) Researchers cautioned that "causality could not be inferred from their cross-sectional study." Explain why this is the case. c) Comment on the title of the article in light of your answer to part b.

Migraines Some people claim they can get relief from migraine headache pain by drinking a large glass of ice water. Researchers plan to enlist several people who suffer from migraines in a test. Participants will be randomly assigned to a standard pain reliever or a placebo. When a participant experiences a migraine headache, he or she will take the pill. Half of each group will also drink ice water. Participants will then report the level of pain relief they experience. a) Identify the factors and levels in this experiment. b) Identify the treatments and the response variable. c) Is there any blinding described in the study? d) No blocking is described in the study. What might be an appropriate variable on which to block? Clearly explain why you think this variable would be appropriate.

Low-cal dog food A dog food company wants to compare a new lower calorie food with their standard dog food to see if it's effective in helping inactive dogs maintain a healthy weight. They have found several dog owners willing to participate in the trial. The dogs have been classified as small, medium, or large breeds, and the company will supply some owners of each size of dog with one of the two foods. The owners have agreed not to feed their dogs anything else for a period of 6 months, after which the dogs' weights will be checked. a) Identify the treatments, the experimental units, and the response variable. b) Describe a method of assigning treatments if this is to be a randomized block design with size of the breed as the blocking variable. c) Is blinding important in this experiment? Doubleblinding? How could blinding be conducted?

Omega-3 An experiment that showed that high doses of omega-3 fats might be of benefit to people with bipolar disorder involved a control group of subjects who received a placebo. Why didn't the experimenters just give everyone the omega-3 fats to see if they improved?

Insomnia Exercise 10 describes an experiment showing that exercise helped people sleep better. The experiment involved other groups of subjects who didn't exercise. Why didn't the experimenters just have everyone exercise and see if their ability to sleep improved?

Omega-3 revisited Exercises 21 describes an experiment investigating a dietary approach to treating bipolar disorder. Researchers randomly assigned 30 subjects to two treatment groups, one group taking a high dose of omega-3 fats and the other a placebo. a) Why was it important to randomize in assigning the subjects to the two groups? b) What would be the advantages and disadvantages of using 100 subjects instead of $30 ?$

Insomnia again Exercises 10 and 22 describe an experiment investigating the effectiveness of exercise in combating insomnia. Researchers randomly assigned half of the 40 volunteers to an exercise program. a) Why was it important to randomize in deciding who would exercise? b) What would be the advantages and disadvantages of using 100 subjects instead of $40 ?$

Omega-3, finis Exercises 21 and 23 describe an experiment investigating the effectiveness of omega-3 fats in treating bipolar disorder. Suppose some of the 30 subjects were very active people who walked a lot or got vigorous exercise several times a week, while others tended to be more sedentary, working office jobs and watching a lot of TV. Why might researchers choose to block the subjects by activity level before randomly assigning them to the omega-3 and placebo groups?

Insomnia, at last Exercises $10,22,$ and 24 describe an experiment investigating the effectiveness of exercise in combating insomnia. Suppose some of the 40 subjects had maintained a healthy weight, but others were quite overweight. Why might researchers choose to block the subjects by weight level before randomly assigning some of each group to the exercise program?

Tomatoes Describe a strategy to randomly split the 24 tomato plants into the three groups for the chapter's completely randomized single factor test of OptiGro fertilizer.

Tomatoes II The chapter also described a completely randomized two-factor experiment testing OptiGro fertilizer in conjunction with two different routines for watering the plants. Describe a strategy to randomly assign the 24 tomato plants to the six treatments.

Shoes A running-shoe manufacturer wants to test the effect of its new sprinting shoe on 100 -meter dash times. The company sponsors 5 athletes who will try out for the 100 -meter dash in the 2016 Summer Olympic games. To test the shoe, it has all 5 runners run the 100 -meter dash with a competitor's shoe and then again with their new shoe. The company uses the difference in times as the response variable. a) Suggest some improvements to the design. b) Why might the shoe manufacturer not be able to generalize the results they find to all runners?

Swimsuits A swimsuit manufacturer wants to test the speed of its newly designed suit. The company designs an experiment by having 6 randomly selected Olympic swimmers swim as fast as they can with their old swimsuit first and then swim the same event again with the new, expensive swimsuit. The company will use the difference in times as the response variable. Criticize the experiment and point out some of the problems with generalizing the results.

Hamstrings Athletes who had suffered hamstring injuries were randomly assigned to one of two exercise programs. Those who engaged in static stretching returned to sports activity in a mean of 15.2 days faster than those assigned to a program of agility and trunk stabilization exercises. (Journal of Orthopaedic $\&$ Sports Physical Therapy 34 [March 2004 ]: 3 ) a) Explain why it was important to assign the athletes to the two different treatments randomly. b) There was no control group consisting of athletes who did not participate in a special exercise program. Explain the advantage of including such a group. c) How might blinding have been used? d) One group returned to sports activity in a mean of 37.4 days $(S D=27.6 \text { days })$ and the other in a mean of 22.2 days $(S D=8.3$ days). Do you think this difference is statistically significant? Explain.

Diet and blood pressure An experiment that showed that subjects fed the DASH diet were able to lower their blood pressure by an average of 6.7 points compared to a group fed a "control diet." All meals were prepared by dieticians. a) Why were the subjects randomly assigned to the diets instead of letting people pick what they wanted to eat? b) Why were the meals prepared by dieticians? c) Why did the researchers need the control group? If the DASH diet group's blood pressure was lower at the end of the experiment than at the beginning, wouldn't that prove the effectiveness of that diet? d) What additional information would you want to know in order to decide whether an average reduction in blood pressure of 6.7 points was statistically significant?

Mozart Will listening to a Mozart piano sonata make you smarter? In a 1995 study published in the journal Psychological Science, Rauscher, Shaw, and Ky reported that when students were given a spatial reasoning section of a standard IQ test, those who listened to Mozart for 10 minutes improved their scores more than those who simply sat quietly. a) These researchers said the differences were statistically significant. Explain what that means in context. b) Steele, Bass, and Crook tried to replicate the original study. In their study, also published in Psychological Science $(1999),$ the subjects were 125 college students who participated in the experiment for course credit. Subjects first took the test. Then they were assigned to one of three groups: listening to a Mozart piano sonata, listening to music by Philip Glass, and sitting for 10 minutes in silence. Three days after the treatments, they were retested. Draw a diagram displaying the design of this experiment. c) These boxplots show the differences in score before and after treatment for the three groups. Did the Mozart group show improvement? (FIGURE CAN'T COPY) d) Do you think the results prove that listening to Mozart is beneficial? Explain.

Full moon It's a common belief that people behave strangely when there's a full moon and that as a result police and emergency rooms are busier than usual. Design a way you could find out whether there is any merit to this belief. Will you use an observational study or an experiment? Why?

Wine A 2001 Danish study published in the Archives of Internal Medicine casts significant doubt on suggestions that adults who drink wine have higher levels of "good" cholesterol and fewer heart attacks. These researchers followed a group of individuals born at a Copenhagen hospital between 1959 and 1961 for 40 years. Their study found that in this group the adults who drank wine were richer and better educated than those who did not. a) What kind of study was this? b) It is generally true that people with high levels of education and high socioeconomic status are healthier than others. How does this call into question the supposed health benefits of wine? c) Can studies such as these prove causation (that wine helps prevent heart attacks, that drinking wine makes one richer, that being rich helps prevent heart attacks, etc.)? Explain.

Swimming Recently, a group of adults who swim regularly for exercise were evaluated for depression. It turned out that these swimmers were less likely to be depressed than the general population. The researchers said the difference was statistically significant. a) What does "statistically significant" mean in this context? b) Is this an experiment or an observational study? Explain. c) News reports claimed this study proved that swimming can prevent depression. Explain why this conclusion is not justified by the study. Include an example of a possible lurking variable. d) But perhaps it is true. We wonder if exercise can ward off depression, and whether anaerobic exercise (like weight training) is as effective as aerobic exercise (like swimming). We find 120 volunteers not currently engaged in a regular program of exercise. Design an appropriate experiment.

Dowsing Before drilling for water, many rural homeowners hire a dowser (a person who claims to be able to sense the presence of underground water using a forked stick). Suppose we wish to set up an experiment to test one dowser's ability. We get 20 identical containers, fill some with water, and ask him to tell which ones they are. a) How will we randomize this procedure? b) The dowser correctly identifies the contents of 12 out of 20 containers. Do you think this level of success is statistically significant? Explain. c) How many correct identifications (out of 20 ) would the dowser have to make to convince you that the forked-stick trick works? Explain.

Healing A medical researcher suspects that giving postsurgical patients large doses of vitamin E will speed their recovery times by helping their incisions heal more quickly. Design an experiment to test this conjecture. Be sure to identify the factors, levels, treatments, response variable, and the role of randomization.

Reading Some schools teach reading using phonics (the sounds made by letters) and others using whole language (word recognition). Suppose a school district wants to know which method works better. Suggest a design for an appropriate experiment.

Gas mileage Do cars get better gas mileage with premium instead of regular unleaded gasoline? It might be possible to test some engines in a laboratory, but we'd rather use real cars and real drivers in real day-to-day driving, so we get 20 volunteers. Design the experiment.

Weekend deaths A study published in the New England Journal of Medicine (Aug. 2001) suggests that it's dangerous to enter a hospital on a weekend. During a 10-year period, researchers tracked over 4 million emergency admissions to hospitals in Ontario, Canada. Their findings revealed that patients admitted on weekends had a much higher risk of death than those who went on weekdays. a) The researchers said the difference in death rates was "statistically significant." Explain in this context what that means. b) What kind of study was this? Explain. c) If you think you're quite ill on a Saturday, should you wait until Monday to seek medical help? Explain. d) Suggest some possible explanations for this troubling finding.

Shingles A research doctor has discovered a new ointment that she believes will be more effective than the current medication in the treatment of shingles (a painful skin rash). Eight patients have volunteered to participate in the initial trials of this ointment. You are the statistician hired as a consultant to help design a completely randomized experiment. a) Describe how you will conduct this experiment. b) Suppose the eight patients' last names start with the letters A to H. Using the random numbers listed below, show which patients you will assign to each treatment. Explain your randomization procedure clearly. $$41098 \quad 18329 \quad 78458 \quad 31685 \quad 55259$$ c) Can you make this experiment double-blind? How? d) The initial experiment revealed that males and females may respond differently to the ointment. Further testing of the drug's effectiveness is now planned, and many patients have volunteered. What changes in your first design, if any, would you make for this second stage of testing?

Beetles Hoping to learn how to control crop damage by a certain species of beetle, a researcher plans to test two different pesticides in small plots of corn. A few days after application of the chemicals, he'll check the number of beetle larvae found on each plant. The researcher wants to know whether either pesticide works and whether there is a significant difference in effectiveness between them. Design an appropriate experiment.

SAT Prep Can special study courses actually help raise SAT scores? One organization says that the 30 students they tutored achieved an average gain of 60 points when they retook the test. a) Explain why this does not necessarily prove that the special course caused the scores to go up. b) Propose a design for an experiment that could test the effectiveness of the tutorial course. c) Suppose you suspect that the tutorial course might be more helpful for students whose initial scores were particularly low. How would this affect your proposed design?

Safety switch An industrial machine requires an emergency shutoff switch that must be designed so that it can be easily operated with either hand. Design an experiment to find out whether workers will be able to deactivate the machine as quickly with their left hands as with their right hands. Be sure to explain the role of randomization in your design.

Washing clothes A consumer group wants to test the effectiveness of a new "organic" laundry detergent and make recommendations to customers about how to best use the product. They intentionally get grass stains on 30 white T-shirts in order to see how well the detergent will clean them. They want to try the detergent in cold water and in hot water on both the "regular" and "delicates" wash cycles. Design an appropriate experiment, indicating the number of factors, levels, and treatments. Explain the role of randomization in your experiment.

Skydiving, anyone? A humor piece published in the British Medical Journal ("Parachute use to prevent death and major trauma related to gravitational challenge: systematic review of randomized control trials,” Gordon, Smith, and Pell, $B M J, 2003: 327$ ) notes that we can't tell for sure whether parachutes are safe and effective because there has never been a properly randomized, double-blind, placebo-controlled study of parachute effectiveness in skydiving. (Yes, this is the sort of thing statisticians find funny.....) Suppose you were designing such a study: a) What is the factor in this experiment? b) What experimental units would you propose? c) What would serve as a placebo for this study? d) What would the treatments be? e) What would the response variable be? f) What sources of variability would you control? g) How would you randomize this "experiment"? h) How would you make the experiment double-blind?