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Open Access Highly Accessed Research article

How many repeated measures in repeated measures designs? Statistical issues for comparative trials

Andrew J Vickers

Author Affiliations

Integrative Medicine Service, Biostatistics Service, Memorial Sloan Kettering Cancer Center, Howard 13, 1275 York Avenue NY, NY 10021, USA

BMC Medical Research Methodology 2003, 3:22  doi:10.1186/1471-2288-3-22

Published: 27 October 2003

Abstract

Background

In many randomized and non-randomized comparative trials, researchers measure a continuous endpoint repeatedly in order to decrease intra-patient variability and thus increase statistical power. There has been little guidance in the literature as to selecting the optimal number of repeated measures.

Methods

The degree to which adding a further measure increases statistical power can be derived from simple formulae. This "marginal benefit" can be used to inform the optimal number of repeat assessments.

Results

Although repeating assessments can have dramatic effects on power, marginal benefit of an additional measure rapidly decreases as the number of measures rises. There is little value in increasing the number of either baseline or post-treatment assessments beyond four, or seven where baseline assessments are taken. An exception is when correlations between measures are low, for instance, episodic conditions such as headache.

Conclusions

The proposed method offers a rational basis for determining the number of repeat measures in repeat measures designs.