“Pecans Lower Cholesterol” is a headline that appeared in the magazine Woman’s World (November 1, 2010). Consider the following five study descriptions. For each of the study descriptions, answer these five questions:
Question 1: Is the described study an observational study or an experiment?
Question 2: Did the study use random selection from some population?
Question 3: Did the study use random assignment to experimental groups?
Question 4: Would the conclusion “pecans lower cholesterol” be appropriate given the study description? Explain.
Question 5: Would it be reasonable to generalize conclusions from this study to some larger population? If so, what population?
Study 1: Five hundred students were selected at random from those enrolled at a large college in Florida. Each student in the sample was asked whether they ate pecans more than once in a typical week, and their cholesterol levels were also measured. The average cholesterol level was significantly lower for the group who ate pecans more than once a week than for the group that did not.
Study 2: One hundred people who live in Los Angeles volunteered to participate in a statistical study. The volunteers were divided based on gender, with women in group 1 and men in group 2. Those in group 1 were asked to eat 3 ounces of pecans daily for 1 month. Those in group 2 were asked not to eat pecans for 1 month. At the end of the month, the average cholesterol level was significantly lower for group 1 than for group 2.
Study 3: Two hundred people volunteered to participate in a statistical study. Each person was asked how often he or she ate pecans, and their cholesterol levels were also measured. The average cholesterol level for those who ate pecans more than once a week was significantly lower than the average cholesterol level for those who did not eat pecans.
Study 4: Two hundred people volunteered to participate in a statistical study. For each volunteer, a coin was tossed. If the coin landed heads up, the volunteer was assigned to group 1. If the coin landed tails up, the volunteer was assigned to group 2. Those in group 1 were asked to eat 3 ounces of pecans daily for 1 month. Those in group 2 were asked not to eat pecans for 1 month. At the end of the month, the average cholesterol level was significantly lower for group 1 than for group 2.
Study 5: One hundred students were selected at random from those enrolled at a large college. Each of the selected students was asked to participate in a study, and all agreed to participate. For each student, a coin was tossed. If the coin landed heads up, the student was assigned to group 1. If the coin landed tails up, the student was assigned to group 2. Those in group 1 were asked to eat 3 ounces of pecans daily for 1 month. Those in group 2 were asked not to eat pecans for 1 month. At the end of the month, the aver-age cholesterol level was significantly lower for group 1 than for group 2.