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.
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- Scruggs, T. E., Mastropieri, M. A., & Casto, G. (1987). The quantitative synthesis of single-subject research: Methodology and validation. Remedial and Special Education, 8(2), 24-33. · DOI 10.1177/074193258700800206
- Parker, R. I., Vannest, K. J., Davis, J. L., & Sauber, S. B. (2011). Combining nonoverlap and trend for single-case research: Tau-U. Behavior Therapy, 42(2), 284-299. · DOI 10.1016/j.beth.2010.08.006
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