The consequence of ignoring a nested factor on measures of effect size in analysis of variance

Although the consequences of ignoring a nested factor on decisions to rejec t the null hypothesis of no treatment effects have been discussed in the li terature, typically researchers in applied psychology and education ignore treatment providers (often a nested factor) when comparing the efficacy of treatments. The incorrect analysis, however, not only invalidates tests of hypotheses, but it also overestimates the treatment effect. Formulas were d erived and a Monte Carlo study was conducted to estimate the degree to whic h the F statistic and treatment effect size measures are inflated by ignori ng the effects due to providers of treatments. These untoward effects are i llustrated with examples From psychotherapeutic treatments.