Issues in the use of change scores in randomized trials

The literature on the measurement of change is often confusing and contradictory. Some authors advocate the use of change scores as the best approach to the analysis of treatment effects in clinical trials; others maintain that change scores should be avoided entirely. This paper reviews these arguments and demonstrates that contradictions arise in part from different definitions of change, and in part from some misunderstanding of the relationship between reliability and responsiveness to change. Conditions under which it is, and is not appropriate to use change scores in experimental designs are specified, and formulae for sample size calculations are introduced.