View article

Twenty-five years ago, we proposed the use of the PND (percentage of nonoverlapping data) statistic for quantitative synthesis (or meta-analysis) of single-subject research. This procedure was controversial from its beginning, with criticism divided between those maintaining that no quantitative method should be used to summarize single-subject research and those suggesting that other methods may be preferable. Since that time, more than 40 research summaries have been published using the PND statistic or its variants, and a smaller number have been published using other methods. We argue that the PND method has proven to be very useful over time for this purpose, though other methods have also contributed. These procedures to date have led to more systematic, objective summaries of single-subject research. We discuss implications of present knowledge for future applications of research synthesis.