Papers in Progress

Papers in Progress

[Under review - Psychological Methods] Last updated on April 20, 2021

Adjusting for Covariates with the Standardized Mean Difference Effect Size

Larry V. Hedges, Elizabeth Tipton, Rrita Zejnullahi, Karina G. Diaz

Adjusting for baseline characteristics (e.g., pre-tests) is common practice when evaluating the effects of interventions. While it has been long-known that the use of these adjustments typically leads to an increase in precision of treatment effect estimates in primary analyses of randomized trials, representing the results of such analyses in the form of effect sizes, however, has received little attention. In this article, we provide a general framework for defining effect sizes in designs with a pre-test and propose estimators of those effects sizes. The estimators and their sampling distributions are evaluated using a simulation study and then demonstrated using an example from published data.