Thursday, April 26, 2012 - 3:00pm to 4:00pm
ISyE Executive Classroom
TITLE: The Effect of Omitted Variables in the Multilevel Mediation Model
SPEAKER: Professor Davood Tofighi, GT School of Psychology
Mediational analysis is a statistical approach that examines the effect of treatment (e.g., prevention) on an outcome (e.g., substance use) achieved by targeting and changing one or more intervening variable(s) (e.g., risk and protective factors such as peer drug use norms). Multilevel (mixed model) mediation analysis examines the indirect effect of an independent variable on an outcome achieved by targeting and changing an intervening variable (mediator) in clustered (multilevel) data. We study analytically and through simulation the effects of an omitted variable at Level 2 (cluster level) on a 1-1-1 multilevel mediation model for a randomized experiment conducted within clusters in which the treatment, mediator, and outcome are all measured at Level 1 (individuals). When the residuals in the equations for the mediator and the outcome variables are fully orthogonal, the two methods of calculating the indirect effect (ab, c – c' ) are equivalent at the between- and within-cluster levels. Omitting a variable at Level 2 changes the interpretation of the indirect effect and will induce correlations between the random intercepts or random slopes. The equality of within-cluster ab and c – c' no longer holds. Correlation between random slopes implies that the within-cluster indirect effect is conditional, interpretable at the grand mean level of the omitted variable.