Overview
Longitudinal studies fail when the measurement wave spacing doesn't match the theoretical change trajectory. If a behavioral intervention produces its maximum effect at 3 months but measurements are taken at baseline and 12 months, the study will underestimate the intervention's peak effect and potentially miss it entirely. The measurement design must be theoretically informed — based on when change is expected to occur, not on what's convenient.
The Longitudinal Study Design Framework aligns wave timing with the theoretical change trajectory, calculates attrition-adjusted sample sizes, plans retention strategies, and selects the analytic approach (growth curve modeling, cross-lagged panel, interrupted time series) that answers the specific longitudinal question.