A nutritionist wants to conduct a study to validate the efficacy of an herb as an aid in weight loss. She randomly assigns half of a group of overweight persons to a treatment group who are given the herb with instructions for its use and a planned diet for six weeks. The other half of the group is given parsley with the same instructions and same diet. A nurse at the nutrition center weighs each subject on Friday of each week. Select the potential source of confounding.

A. Placebo effect
B. Experimenter effect
C. Method of assignment to treatment and control groups
D. The study is essentially free of potential confounding.

My answer is B, but I think it could also be D

Does the nurse know who is in which group? If so, B is correct.

It is D

The potential source of confounding in this study is option C - the method of assignment to treatment and control groups.

When conducting a study, it is important to ensure that the treatment and control groups are assigned randomly to minimize the likelihood of confounding variables. In this case, the nutritionist randomly assigned half of the overweight persons to the treatment group and the other half to the control group.

However, there is still a potential for confounding based on how the random assignment was carried out. If, for example, the nutritionist used a flawed randomization method or if there was some bias in the selection process, then the two groups may not be truly equal in terms of characteristics that could influence weight loss.

Option B - experimenter effect refers to the potential bias or influence the nutritionist may have on the results. For instance, if the nutritionist unconsciously treats the treatment group differently or provides additional guidance or encouragement to the treatment group, it could introduce a confounding factor. However, it is not explicitly mentioned in the scenario, so it may not be a significant concern.

Option A - placebo effect refers to the psychological and physiological effects that can occur when people receive a placebo (inactive substance) and believe they are receiving an active treatment. In this study, both groups are receiving treatments (herb and parsley), so the placebo effect is not a potential confounding factor.

Option D suggests that the study is essentially free of potential confounding, but given the possibility of confounding arising from the method of assignment, this option is not accurate.

Therefore, the correct answer is C - the method of assignment to treatment and control groups.