Which of the following factors influences statistical power in a cohort study?

• A. Expected effect size
• B. Follow-up duration
• C. Number of events
• D. Variability

Answer: (C)

In cohort studies, the amount of information you have about the exposure–outcome relationship comes from the events you observe. The more events (outcomes) you see, the more precise your estimate of the association, and the greater your power to detect a real effect if one exists. When events are few, random variation overwhelms the signal, making it hard to distinguish a true effect from noise even if there is one.

Other factors influence power but often through their impact on available information. A larger true effect size makes detection easier, longer follow-up can yield more events, and higher variability or measurement error adds noise and lowers power. Yet the direct lever is the number of observed events: more events mean more information and higher power, especially important for rare outcomes or short follow-up.