Section 1.3 Design of Experiments 25 b. The subjects in the low-exercise group had considerably higher blood pressure, on the average, than subjects in the high-exercise group. The researcher concluded that exercise decreases blood pressure. Is this conclusion well justified? Explain. 19. A medical researcher wants to determine whether exercising can lower blood pressure. She recruits 100 people with high blood pressure to participate in the study. She assigns a random sample of 50 of them to pursue an exercise program that includes daily swimming and jogging. She assigns the other 50 to refrain from vigorous activity. She measures the blood pressure of each of the 100 individuals both before and after the study. a. Is this a randomized experiment or an observational study? b. On the average, the subjects in the exercise group substantially reduced their blood pressure, while the subjects in the no-exercise group did not experience a reduction. The researcher concluded that exercise decreases blood pressure. Is this conclusion well justified? Explain. 20. An agricultural scientist wants to determine the effect of fertilizer type on the yield of tomatoes. There are four types of fertilizer under consideration. She plants tomatoes on four plots of land. Each plot is treated identically except for receiving a different type of fertilizer. a. What are the treatments? b. Is this a randomized experiment or an observational study? c. The yields differ substantially among the four plots. Can you conclude that the differences in yield are due to the differences in fertilizer? Explain. Working with the Concepts 21. Air pollution and colds: A scientist wants to determine whether people who live in places with high levels of air pollution get more colds than people in areas with little air pollution. Do you think it is possible to design a randomized experiment to study this question, or will an observational study be necessary? Explain. 22. Cold medications: A scientist wants to determine whether a new cold medicine relieves symptoms more effectively than a currently used medicine. Do you think it is possible to design a randomized experiment to study this question, or will an observational study be necessary? Explain. 23. Taxicabs and crime: A sociologist discovered that regions that have more taxicabs tend to have higher crime rates. Does increasing the number of taxicabs cause the crime rate to increase, or could the result be due to confounding? Explain. 24. Recovering from heart attacks: In a study of people who had suffered heart attacks, it was found that those who lived in smaller houses were more likely to recover than those who lived in larger houses. Does living in a smaller house increase the likelihood of recovery from a heart attack, or could the result be due to confounding? Explain. 25. Eat your vegetables: In an observational study, people who ate four or more servings of fresh fruits and vegetables each day were less likely to develop colon cancer than people who ate little fruit or vegetables. True or false: a. The results of the study show that eating more fruits and vegetables reduces your risk of contracting colon cancer. b. The results of the study may be due to confounding, since the lifestyles of people who eat large amounts of fruits and vegetables may differ in many ways from those of people who do not. 26. Vocabulary and height: A vocabulary test was given to students at an elementary school. The students’ ages ranged from 5 to 11 years old. It was found that the students with larger vocabularies tended to be taller than the students with smaller vocabularies. Explain how this result might be due to confounding. 27. Secondhand smoke: A recent study compared the heart rates of 19 infants born to nonsmoking mothers with those of 17 infants born to mothers who smoked an average of 15 cigarettes a day while pregnant and after giving birth. The heart rates of the infants at one year of age were 20% slower on the average for the smoking mothers. a. What is the outcome variable? b. What is the treatment variable? c. Was this a cohort study or a case-control study? d. Was the study prospective, cross-sectional, or retrospective? e. Could the results be due to confounding? Explain. Source: Environmental Health Perspectives 118:a158–a159 28. Pollution in China: In a recent study, Z. Zhao and colleagues measured the levels of formaldehyde in the air in 34 classrooms in the schools in the city of Taiyuan, China. On the same day, they gave questionnaires to 1993 students aged 11–15 in those schools, asking them whether they had experienced respiratory problems (such as asthma attacks, wheezing, or shortness of breath). They found that the students in the classrooms with higher levels of formaldehyde reported more respiratory problems. a. What is the outcome variable? b. What is the treatment variable? c. Was this a cohort study or a case-control study? d. Was the study prospective, cross-sectional, or retrospective? e. Could the results be due to confounding? Explain. Source: Environmental Health Perspectives 116:90–97
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