Adjusting for a Mediator in Models With Two Crossed Treatment Variables
研究了在2×2双因素设计中，当中介变量受主效应或交互效应影响时，标准协方差分析调整中介变量会导致参数估计偏误，并通过解析和模拟验证。
In a simple mediation model, the effect of a manipulated variable X on a dependent variable Y over and above the effect of the mediator Me can be estimated by regressing Y on X and Me. The impact of X on Y in such a model is adjusted for the relationship both between X and Me and between Me and Y. The authors examine the adjustment function in the context of a 2 × 2 design with two manipulated variables. In such a situation, the mediator could be affected by either one of the main effects and/or the interaction between two manipulated variables. To adjust for the impact of the mediator, a standard procedure has been to rely on an ANCOVA that includes only the mediator. The authors show, both analytically and with simulations, that this leads to improper control of the mediator and to biased estimates of the model parameters.