In cohort studies of a suspected etiologic factor, which condition is essential to minimize bias?

• A. The exposed and nonexposed groups be as similar as possible with regard to possible confounding factors
• B. There be equal numbers of persons in both study groups
• C. At the beginning of the study, those with the disease and those without the disease have equal risk of having the factor
• D. The study group with the factor and the study group without the factor be representative of the general population

Answer: (A)

In cohort studies, bias is most effectively minimized when the exposed and unexposed groups are as similar as possible in terms of confounding factors. Since the exposure isn’t assigned randomly, differences in baseline characteristics like age, sex, socioeconomic status, or other risk factors can influence the outcome independently of the exposure, leading to a distorted estimate of the true association. By making the groups comparable on these factors—through matching, restricting the study to certain confounder levels, or adjusting for confounders in analysis—you reduce the chance that these other variables explain any observed difference in outcomes. Equal numbers in each group don’t guarantee similarity on important characteristics, and having equal baseline risk of the factor by disease status or aiming for representativeness of the general population mainly affects generalizability rather than internal bias.