Abstract
Health Related Quality of Life (HRQoL) measures are becoming more frequently used in clinical trials and health services research, both as primary and secondary endpoints. Investigators are now asking statisticians for advice on how to plan and analyse studies using HRQoL measures, which includes questions on sample size. Sample size requirements are critically dependent on the aims and objectives of the study, the proposed summary measure and effect size and the method of calculating the test statistic.
We present a tutorial on methods of sample size calculation for HRQoL outcomes. We also briefly review the HRQoL literature to see what has been done in practice. The aim of this tutorial is provide pragmatic guidance to researchers on the methods of calculating sample sizes when using HRQoL measures as outcomes.
HRQoL measures such as the SF-36, NHP and QLQ-C30 are usually measured on an ordered categorical (ordinal) scale. We argue that it is often incorrect to treat the scales as if they were continuous and normally distributed and that the mean score may not be a good summary measure of HRQoL data. However the ordinal scaling of HRQoL measures leads to problems in determining sample size, and we suggest that the odds ratio (OR) may be a more suitable summary measure for comparing groups (rather than the mean difference) and therefore methods suitable for ordinal data be used for analysis.
The frequency distribution of HRQoL scores should be assessed to see if parametric assumptions are satisfied and whether or not the sample mean is a good summary measure of the data. Given the non-normal distribution of the majority HRQoL outcome measures, summary measures such as means and standard deviations are difficult to interpret. Thus standardised differences (effect sizes) and parametric methods may not be a suitable basis for calculation of sample size. Finally we argue, that any sample size calculation (with all its attendant assumptions) leads to better research than no sample size calculation at all.
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Walters, S.J., Campbell, M.J. & Paisley, S. Methods for Determining Sample Sizes for Studies Involving Health-Related Quality of Life Measures: A Tutorial. Health Services & Outcomes Research Methodology 2, 83–99 (2001). https://doi.org/10.1023/A:1020102612073
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DOI: https://doi.org/10.1023/A:1020102612073