THE COUNTERNULL VALUE OF AN EFFECT SIZE: A New Statistic

Abstract

We introduce a new, readily computed statistic, the counternull value of an obtained effect size, which is the nonnull magnitude of effect size that is supported by exactly the same amount of evidence as supports the null value of the effect size. In other words, if the counternull value were taken as the null hypothesis, the resulting p value would be the same as the obtained p value for the actual null hypothesis. Reporting the counternull, in addition to the p value, virtually eliminates two common errors: (a) equating failure to reject the null with the estimation of the effect size as equal to zero and (b) taking the rejection of a null hypothesis on the basis of a significant p value to imply a scientifically important finding. In many common situations with a one-degree-of-freedom effect size, the value of the counternull is simply twice the magnitude of the obtained effect size, but the counternuU is defined in general, even with multidegree-of-freedom effect sizes, and therefore can be applied when a confidence interval cannot he. The use of the counternull can be especially useful in meta-analyses when evaluating the scientific importance of summary effect sizes.