The use of effect size measures to bolster research findings has increased over the past 15 years. In general, the inclusion of effect sizes is encouraged by many in the scientific community to increase the magnitude of research findings. However, the inclusion of effect size calculations has become an area of controversy amongst scholars who utilize single-case research designs. This paper discusses the overall construct of effect sizes, the controversy of effect size metrics as it relates to single-case research, and an examination of the utility of one commonly used effect size metric in single-case designs: the percentage of non-overlapping data.