The utility of the PND statistic: a reply to Allison and Gorman.

Quantitative synthesis ("meta-analysis") of single-subject research has rarely been conducted, partly because of a lack of agreement on how study outcomes are to be quantified. This article provides a response to Allison and Gorman (Behaviour Research and Therapy, 31, 621-631, 1993), who listed some problematic characteristics of use of the PND (percent of non-overlapping data) statistic for computing single-subject study outcomes, and recommended a regression-based solution to computation of effect sizes from single-subject research reports. Although Allison and Gorman are generally accurate in pointing out some limitations of the use of the PND statistic, they have been less thorough in identifying its relative strengths. Among these strengths is the fact that the PND statistic and its variations (a) have been shown to be strongly related to qualitative, "expert" ratings, (b) have been successfully employed in at least seven separate integrative reviews, and (c) have produced results which are complementary to more qualitative reviews of the same literature. In contrast, Allison and Gorman did not report results of applications of their procedure and, although their procedure has apparent theoretical support, it may be less useful in synthesizing existing single-subject literature.