Planning a longitudinal study. II. Frequency of measurement and study duration.

Two IMPORTANT issues in planning and executing a longitudinal study are the frequency of repeated measurements on the subjects and the study duration. The scientific goals of a study, physical resources, available staff and funds will contribute to an investigator’s intuition regarding the proper balance of these two factors. Statistical techniques can aid in planning and in evaluating choices of frequency of measurement and study duration. They assume that characterizing individual change in function with age (time) is an important component of a longitudinal study.