A simple, general purpose display of magnitude of experimental effect.

Abstract

We introduce the binomial effect size display (BESD), which is useful because it is (a) easily understood by researchers, students, and lay persons; (b) widely applicable; and (c) conveniently computed. The BESD displays the change in success rate (e.g., survival rate, improvement rate, etc.) attributable to a new treatment procedure. For example, an r of .32, the average size of the effect of psychotherapy, is said to account for only 10% of the variance; however, the BESD shows that this proportion of variance accounted for is equivalent to increasing the success rate from 34% to 66%, which would mean, for example, reducing an illness rate or a death rate from 66% to 34%. Traditionally, behavioral researchers have concentrated on reporting significance levels of experimental effects. Recent years, however, have shown a welcome increase in emphasis on reporting the magnitude of experimental effects obtained (Cohen, 1977; Fleiss, 1969; Friedman, 1968, Glass, Note 1, Hays, 1973; Rosenthal, 1978; Rosenthal & Rubin, 1978; Smith & Glass, 1977).