Current research uses effect size guidelines to determine if there is a small, medium, or large effect for group design interventions. A best practice methodology and guidelines for determining the effect size of an intensive intervention in a single-subject design have not been established. The present study identifies cut score guidelines for single-subject research designs to determine if the intervention had a small, medium, or large effect. This study examines the distribution of effect size, using the Percentage of Non-Overlapping Data (PNOD) and Cohen’s d, for a large sample of intensive academic interventions. In addition, differences in distribution for varying environmental settings (e.g., clinic-based and school-based) and targeted skill areas (e.g., math, reading, and writing) are examined. This information will assist interventionists in selecting interventions, determining if an intervention is working, and if changes to an intervention should be considered.