Analysis of repeated measurements in physical therapy research

In this paper, I attempt to introduce physical therapists to the most common statistical tests for analysing differences between repeated measurements over time. Using the example of 'whole-body flexibility' recorded at six different times of day and the Statistical Package for the Social Sciences (SPSS), I discuss the advantages and disadvantages of the various approaches for analysing a simple one-factor design. The most important issues in test selection for repeated measures are the exploration of, and correction for, the violation of 'sphericity' when employing a univariate general linear model (GLM), as well as the sample size when adopting a multivariate GLM. I summarize current advice on choice of test with the aid of a 'decision tree', based on the results of documented statistical simulations which have investigated how the various statistical tests 'perform' in certain situations. Lastly, I comment on the most appropriate ways to present and interpret data drawn from serial measurements.