All Sample Questions

Selection Bias 1

Selection bias


Vignette: A researcher is conducting a study to determine the effectiveness of a new drug in treating hypertension. He enrolled 1000 patients and randomly assigned them to either the treatment group or the placebo group. At the end of the study, the researcher found that the new drug significantly reduced blood pressure in the treatment group compared to the placebo group. However, the researcher also noted that the participants in the treatment group were generally healthier and had better dietary habits than those in the placebo group.

Question: What kind of bias most likely affected the study's results?


A. Selection bias

B. Information bias

C. Confounding bias

D. Observer bias

E. Publication bias


C. Confounding bias


The correct answer is confounding bias. In this study, the health status and dietary habits of participants, which might themselves influence blood pressure, were not equally distributed between the treatment and placebo groups. These variables, therefore, confound the relationship between the treatment and the outcome, potentially leading to a false conclusion about the effectiveness of the new drug. This is known as confounding bias.

Selection bias (A) occurs when there is a systematic difference in characteristics between those who are selected for study and those who are not. Information bias (B) occurs when there is a systematic difference in the collection of data regarding study participants. Observer bias (D) occurs when researchers' expectations influence the outcome of the study. Publication bias (E) occurs when research with positive results is more likely to be published and cited than research with negative results.


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