How many repeated measures in repeated measures designs? Statistical issues for comparative trials.
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.
Vickers
Andrew J
AJ
Integrative Medicine Service, Biostatistics Service, Memorial Sloan Kettering Cancer Center, Howard 13, 1275 York Avenue NY, NY 10021, USA. vickersa@mskcc.org
D018849:Controlled Clinical Trials as Topic; D023381:Endpoint Determination; D006801:Humans; D015233:Models, Statistical; D015203:Reproducibility of Results; D018401:Sample Size