Lesa Hoffman posted on Tuesday, September 30, 2008 - 8:36 pm
I have a proposed two-level model for a dichotomous outcome. I do not expect random slopes, only a random intercept. I am interested in multiple mediational effects (e.g., tests of indirect effects such as X --> M --> Y) at the between level. In reading through the version 5 manual, it appears I cannot use the MODEL INDIRECT command to test indirect effects when numeric integration is required. Further, bootstrapped SEís and CIís (that are often recommended when examining indirect effects) are not available with TYPE=TWOLEVEL.
Given that I am trying to use a two-level model for a dichotomous outcome, is there any kind of estimator or alternative procedure I can use in Mplus v. 5.1 in order to obtain formal tests of indirect effects (and contrasts between indirect effects) at the between level? I suppose one could model the level-2 aggregates of level-1 variables in a single-level analysis (although the outcome would still be dichotomous), but I wanted to see if there was a more principled way of doing this within the two-level procedure.
Thank you for your prompt and helpful response. I have tried to use the MODEL CONSTRAINT command to define an indirect effect as suggested (relevant part of syntax):
%BETWEEN% Y ON X*; M ON X* (XM); Y ON M* (MY);
MODEL CONSTRAINT: NEW (med*0); med=XM*MY;
The estimate for the 'med' indirect effect in the output is not the value from multiplying the estimated XM (=.357)and MY (=.459) effects together (=.164), but something much smaller (=-.012). Can you tell what I am doing wrong?