Comparing Models of Change to Estimate the Mediated Effect in the Pretest–Posttest Control Group Design

Abstract

Models to assess mediation in the pretest–posttest control group design are understudied in the behavioral sciences even though it is the design of choice for evaluating experimental manipulations. The article provides analytical comparisons of the four most commonly used models to estimate the mediated effect in this design: analysis of covariance (ANCOVA), difference score, residualized change score, and cross-sectional model. Each of these models is fitted using a latent change score specification and a simulation study assessed bias, Type I error, power, and confidence interval coverage of the four models. All but the ANCOVA model make stringent assumptions about the stability and cross-lagged relations of the mediator and outcome that might not be plausible in real-world applications. When these assumptions do not hold, Type I error and statistical power results suggest that only the ANCOVA model has good performance. The four models are applied to an empirical example.

Keywords:

• longitudinal mediation
• mediation
• pretest–posttest design
• two waves

ACKNOWLEDGMENTS

The authors would like to acknowledge Leona Aiken, Craig Enders, and Stephen West for their invaluable feedback on the ideas presented in this article.

FUNDING

This research was supported in part by the National Institute on Drug Abuse grant number R37 DA09757.

Notes

¹ Additionally, an ANCOVA model could be fitted that constrains either the b_m2y1 or b_y2m1 or both to zero. These constraints can be made and the fit of these alternative models can be compared to the saturated ANCOVA model. If a model with either or both of these paths constrained to zero fit the data well, the following difference score, residualized change score, and cross-sectional models can be compared to this unsaturated ANCOVA model as opposed to the saturated ANCOVA model.

Additional information

Funding

This research was supported in part by the National Institute on Drug Abuse grant number R37 DA09757.