Subject bias can occur in an experiment when the participants are aware of the purpose of the study and respond in a way that they think the researchers want. For example, if participants believe they are in a study to test if coffee improves your mood, those who drank coffee might claim to be happier than they really are.

Based on the information given, which of the following experiments is most likely to have the issue of a control or placebo that could introduce subject bias?

Select one:

a. A researcher wanted to test how medicine M would lower blood sugar levels of rabbits. She had 20 similar rabbits and randomly divided them into two groups of 10 rabbits each. Rabbits in the treatment group were injected with medicine M, while rabbits in the control group just stayed in the cage. For each rabbit, blood sugar levels were measured before and after the treatment, and the change was computed.

b. To study the effect of medicine A on the treatment of children with tic disorders, 50 patients were randomly divided into two groups of 25 patients each. Patients in the treatment group took medicine A, while patients in the control group took a placebo. The medicine A and placebo were similar in appearance so that patients were not able to figure out which they had taken. After six months, the level of improvement for each patient was evaluated by an independent organization.

c. A company wanted to know how background music could affect the impression of a television commercial on audiences. They conducted an experiment in which 200 people took part. During the experiment, the subjects were asked to sit before screens to take a computer-based test simultaneously. In the test, a television advertisement was played once and questions about the product were asked after the advertisement disappeared. Advertisements and questions were the same for all the subjects except for the background music in the advertisement. Two different versions of background music were randomly selected by computers with equal probabilities. The performance of each subject on answering questions was recorded.

d. A middle school conducted an experiment to show how an advanced teaching method that was different from what was currently employed could improve the academic performances of 8th grade students. Students were informed of the experiment in an assembly. The researchers then randomly selected 15% of all the 8th grade students to form special classes. Students in the special classes were taught using the new approach, while other students remained in their original classes and were taught using the same way as before. General conditions such as class sizes were similar among all classes. After one semester, the changes in academic performances were recorded.