Grape juice and blood pressure. Researchers who wanted to see if drinking grape
ID: 3275366 • Letter: G
Question
Grape juice and blood pressure. Researchers who wanted to see if drinking grape juice could help people lower their blood pressure got 120 non-smokers to volunteer for a study. They measured each person's blood pressure and then randomly divided the subjects into two groups. One group drank a glass of grape juice every day while the other did not. After sixty days the researchers measured everyone's blood pressure again. They reported that differences in changes in blood pressure between the groups were not statistically significant. (a) Was this an experiment or an observational study? Explain briefly. (b) Briefly explain what "not statistically significant" means in this context. (c) Briefly explain why the researchers randomly assigned the subjects to the groups. (d) Since everyone's blood pressure was measured at the beginning and at the end of the study, the researchers could have simply looked at the grape juice drinkers to see if their blood pressure changed. Briefly explain why the researchers bothered to include the control group. (e) Briefly explain why the researchers studied only non-smokers. (f) Other researchers now plan to replicate this study using both smokers and non-smokers. Briefly describe the design strategy you think they should use.Explanation / Answer
SOLUTION -
a) Experiment - There was an application of a treatment, grape juice, to groups containing randomly assigned subjects with a comparison of the blood pressure across the treatment groups.
b) The reduction in blood pressures is not large enough. We cannot justify ruling out the possibility that the differences in changes in blood pressure are due only to the chance variation in the random assignment. In other words, the reduction in blood pressure can likely be attributed to the chance variation in random assignment.
c) Random assignment allows them to minimize the effects of unknown or uncontrollable sources of variation, making two groups that are nearly equal.
d) The control group provided a basis for comparison after the sixty days to determine if the group drinking grape juice had lower blood pressure because of the juice. Maybe everyone's blood pressure naturally drops at a certain time of the year.
e) By studying only non-smokers the researchers were trying to reduce any impact the smoking might have had on blood pressure. The researchers were trying to remove any confounding the smoking might have made on blood pressure.
The new study must be blocked by whether a person smokes or not. Half of each group would then be randomly assigned to the grape juice or no grape juice groups.
f) Block by smoking status, randomly assigning people in each block to the treatments. Compare blood pressure after 60 days within each block.
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