Over the last 10 years, numerous authors have proposed effect size estimators for single-case designs. None, however, has been shown to be equivalent to the usual between-groups standardized mean difference statistic, sometimes called d. The present paper remedies that omission. Most effect size estimators for single-case designs use the within-person variance as the standardization factor for the effect size. But in the usual between-groups study, like a randomized experiment, the between-person variance is the standardization factor. Taking advantage of the fact that most studies that use single-case design methodology report results from more than one case, the authors show how a d-statistic can be developed that is equivalent to the between-groups estimator. The new d-estimator is complex, requiring knowledge of the autocorrelation, the within-person variability, and the between-person variability.