Why is adequate follow-up important in cohort studies?

• A. To minimize attrition bias
• B. To ensure immediate results
• C. To reduce measurement error
• D. To avoid needing ethical approval

Answer: (A)

In cohort studies, keeping track of participants over time is essential because loss to follow-up can distort the observed association. If people drop out and the likelihood of dropout relates to either the exposure or the chance of developing the outcome, the remaining participants may not represent the original groups. This differential loss leads to attrition bias, where incidence rates and the estimated effect are biased by who stayed in the study rather than by the true relationship between exposure and outcome. Adequate follow-up helps keep the groups comparable, preserves the study’s statistical power, and yields more valid estimates of how exposure influences risk. It’s not about getting results immediately, it doesn’t directly reduce measurement error, and ethical approval is a separate consideration.