Guidelines to inform research evidence standards have acknowledged that there is currently no agreed-upon method for treatment effect size estimation in single-case research. This study has examined the application of treatment effect size calculations to supplement visual analysis in single-case research designs (SCD) for participants with autism spectrum disorder (ASD). Ethical considerations for researchers regarding the collection of baseline data in light of behaviors often associated with ASD are discussed. The adequacy of the volume of data points from baseline and treatment phases was explored, and the conclusion had drawn that the majority of studies were not suitable for regression calculations. The median length of total data series was also explored, and the suitability of three nonparametric hand calculations, percentage of nonoverlap (PND), percentage of all nonoverlapping data (PAND), and nonoverlap of all pairs (NAP), is discussed in this light.