The use of nonparametric effect sizes in single study musculoskeletal physiotherapy research: A practical primer

There is a strong push for the inclusion of effect size indexes alongside the reporting of statistical analysis in academic journals. Nonparametric methods of analysis have generally been developed less than their parametric counterparts have, and are also generally less well known. Too often researchers use parametric statistics where nonparametric measures would be more appropriate. This holds true for nonparametric measures of effect size, where even when researchers use nonparametric statistics, some use parametric effect size measures to interpret the result. This paper attempts to provide a practical overview and illustration of the correct usage and interpretation of effect size measures for nonparametric statistics for single study designs using real-world physiotherapy data in the worked examples. This primer covers a range of different formulae based on categorical measures of effect size, as well as between- and within-group designs using ranked data. While this primer does use examples focusing on physiotherapy research, the applications of the information can be used in any field of research.