Guidelines for the design of clinical trials with longitudinal outcomes
by
Sally Galbraith
NHMRC Clinical Trials Centre, University of Sydney
Coauthors: Ian Marschner

A common objective of longitudinal clinical trials is to compare rates of change in a continuous response variable between two groups. The power realized for such a study is a function of both the number of participants recruited and the planned number of measurements for each participant. By varying these two quantities in opposite directions, power can be kept at the desired level. This talk considers the problem of how best to choose the sample size and frequency of measurement, with a view to minimizing either the total number of measurements or the cost of a study. Some general guidelines are first developed for the situation in which all participants have complete observations. In practice, however, longitudinal studies often suffer from dropout, where a participant leaves the study permanently so that no further observations are possible. This leads to a consideration of the impact of unanticipated dropout on power and also ways of allowing for dropout at the design stage. Based on the results of this analysis, some general design guidelines are suggested for longitudinal trials comparing rates of change when dropout is present.

Date received: March 25, 2002