A longitudinal study follows the same people for thirty years; a cross-sectional study compares different age groups on one afternoon. Each design's main strength is the other's main weakness. What is the trade?

B. Longitudinal holds the person constant; cross-sectional confounds cohort.

A longitudinal study compares each person with themselves, so generation cancels, it measures change rather than difference. The cost is not merely time: attrition is selective toward poorer health, so the surviving sample looks better than the cohort it started as, which can hide the very decline the study was built to detect.