Is Busk and Serlin’s measure of therapy effect size d a suitable measure for use in therapy studies? Evidence from simulations

Howard, David and Rose, Miranda (2009) Is Busk and Serlin’s measure of therapy effect size d a suitable measure for use in therapy studies? Evidence from simulations. [Clinical Aphasiology Paper]

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Abstract

Beeson & Robey (2006) advocate Busk & Serlin’s d as a good measure of effect size, suitable for meta-analysis of single case experimental designs of therapy. Unfortunately there are no available data supporting the use of this measure. We propose a set of criteria for a suitable measure of effect size and investigate whether d meets them in 1360 simulations each involving 10000 measures of d. We show that d always overestimates the true effect size and this is primarily related to the number of items in the baseline series and the degree of lag 1 autocorrelation in the series. We show that Busk & Serlin’s d does not meet any of our suggested criteria. It is difficult to see how its use can be justified.

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