Percentage of Non-Overlapping Data

The percentage of non-overlapping data (PND) is a simple nonoverlap effect-size index for single-case (single-subject) research, introduced by Scruggs, Mastropieri, and Casto in 1987 to enable quantitative synthesis of studies that report data graphically rather than as group statistics. PND quantifies how strongly an intervention shifted a behavior by computing the percentage of intervention-phase data points that exceed the single most extreme baseline data point in the direction of desired change. Because it requires only the graphed data and a ruler, PND became one of the most widely used effect sizes in meta-analyses of single-case research in special education, rehabilitation, and behavior analysis. Its very simplicity, however, brings well-documented weaknesses — a ceiling at 100 percent, extreme sensitivity to a single outlying baseline point, and blindness to trend — which motivated later nonoverlap-plus-trend indices such as Parker and colleagues' 2011 Tau-U. PND is best understood as a fast, interpretable, but coarse first look at single-case effect magnitude.