Questions: In a test of the effects of eating a vegetarian diet on cholesterol, one group of 55 randomly selected subjects undergoes a treatment consisting of a vegetarian diet while another group of 58 randomly selected subjects eats a non-vegetarian diet. The researchers doing the measurements of cholesterol are aware of which participants receive the treatment and which do not receive it. Identify any problems that are likely to cause confounding and explain how the problems could be avoided. Identify any problems that are likely to cause confounding. Select all that apply. A. There is likely to be an experimenter effect in this study because the researcher making the cholesterol measurements will influence the subjects by knowing which participants eat a vegetarian diet and which participants do not. B. The groups are so small that confounding is likely to be introduced by the select of the group subjects. C. If there are differences in the measurements of cholesterol, there is no way to know if the differences are attributable to the treatment (eating a vegetarian diet) or to the height of the participant. D. There is likely to be a placebo effect in this study because the subjects know whether they are eating a vegetarian or non-vegetarian diet. E. There is no control group. Explain how the problems could be avoided. Select all that apply. A. The size of the groups should be increased to eliminate the possibility of a biasє sample. B. Differences in the measurements of cholesterol can be correctly attributed to eating vegetarian by giving the vegetarian diet treatment to half of the short subjects and half of the tall subjects and by not giving the vegetarian diet treatment to the other subjects. C. Unfortunately, there is no way to eliminate the placebo effect in this study. D. The study's subjects should not be aware whether they receive the vegetarian diet treatment or not. E. The researchers doing the cholesterol measurements should not be aware of which participants eat a vegetarian diet or do not eat a vegetarian diet.

The answer is A and D:

A. There is likely to be an experimenter effect in this study because the researcher making the cholesterol measurements will influence the subjects by knowing which participants eat a vegetarian diet and which participants do not. D. There is likely to be a placebo effect in this study because the subjects know whether they are eating a vegetarian or non-vegetarian diet.

Explanation for each option:

A. Correct. The experimenter effect can occur when the researchers' knowledge of which participants are in the treatment group influences their measurements or interactions with the subjects. This can introduce bias into the results.

B. Incorrect. While small sample sizes can introduce variability and reduce the power of the study, they do not necessarily cause confounding. Confounding occurs when an outside variable influences both the independent and dependent variables, leading to a false association.

C. Incorrect. The height of the participants is not a relevant confounding variable in this context unless there is a known relationship between height and cholesterol levels that is not being controlled for. The primary concern should be other dietary habits, lifestyle factors, or genetic predispositions.

D. Correct. The placebo effect can occur when participants' knowledge of their treatment influences their outcomes. In this case, knowing whether they are on a vegetarian or non-vegetarian diet could affect their behavior or psychological state, which in turn could influence cholesterol levels.

E. Incorrect. There is a control group in this study, which is the group of participants eating a non-vegetarian diet. The issue is not the absence of a control group but the potential biases introduced by the knowledge of the treatment.

To avoid these problems:

A. Incorrect. Increasing the size of the groups can help reduce variability and increase the power of the study, but it does not directly address the confounding issues related to experimenter or placebo effects.

B. Incorrect. While stratifying by height could control for height as a confounding variable, it is not relevant to the primary concerns of experimenter and placebo effects in this study.

C. Incorrect. There are ways to minimize the placebo effect, such as blinding the participants to their treatment group.

D. Correct. Blinding the subjects so they do not know whether they are receiving the vegetarian diet treatment can help reduce the placebo effect.

E. Correct. Blinding the researchers who measure cholesterol levels so they do not know which participants are in the treatment group can help reduce the experimenter effect.

In summary, the primary issues are the experimenter effect and the placebo effect, which can be mitigated by blinding both the participants and the researchers.