Minimum Sample Size Requirements for a Validation Study of The Schizophrenia Quality of Life Scale-Revision 4 (SQLS-R4)

Abstract

The Schizophrenia Quality of Life Scale-Revision 4 (SQLS-R4) is a widely used self-report quality of life measure used in a broad range of clinical contexts, from primary research to clinical trials. International use of the measure has led to translated versions validated for local context. Most translation and validation studies of the SQLS-R4 have been conducted with modest N, at the threshold of acceptability of even the most liberal recommendations for validation studies. Given the comparatively large number of items in the SQLS-R4 (N=33), low N studies could potentially be underpowered limiting validity and reliability. Using sample sizes from published studies as a baseline, the current investigation sought to determine a minimum sample size for an SQLS-R4 translation/validation study. Methods: A model specification based on the two-factor structure of the SQLS-R4 was constructed to calculate an acceptable model fit based on the sample size used in most SQLS-R4 translation/validation studies (N=100). A series of Monte Carlo simulations was then conducted to determine the sample size required to offer a good fit to data for an adequately powered study. Results: The series of simulations conducted suggests that a minimum sample size for an adequately powered validation/translation study of the SQLS-R4 to provide a good fit to data is N=160. Conclusion: Sample size determination of SQLS-R4 validation/translation studies should be informed by the intrinsic measurement characteristics of the measure to ensure an adequately powered study.