I have a research design with 3 predictors and 2 mediators, tested on 2 different samples with multiple regression analysis. For my first sample (N=70) the results were:variables(mediators) entered at the second step predicted significantly the outcome, but only one of them had a significant coefficient. The IV's had non significant coefficients at step 2, and the beta's decreased in size. In my second sample (N=100), same set of variables, at step 2 the proposed mediators didn't predict significantly the outcome and none had significant coefficients. The IV's also had non significant coefficients at step 2 and they decreased in size. My question – Is mediation supported for the first sample? What about my second sample, what can I say about the results if none of the mediators was significant, can I draw a valid conclusion and still use a mediational design?
All the variables are positivelly correlated and there may be some multicollinearity problems for one of the mediators in each study. Some of the predictors have high correlations with the mediators, above .6.
Best Answer
I have a comment and hopefully an answer.
You use the term "second step", which is a term typically reserved for hierarchical models (e.g., hierarchical regression). Are you certain you are doing mediation analysis? It may help if you describe your question and perhaps how you performed your analyses (e.g., using path analysis, a macro in SPSS). Note that since you have two mediations in your proposed model, it sounds like you would be doing a multi-mediational model, which is something relatively advanced which would likely require path analysis, a programming language, or a macro.
I think the answer to your question is "no", assuming you are doing meditational analysis. For a mediation to be significant, you would need to have a significant direct effect between your mediator and DV, and a significant effect between your IV and mediator, at the very least and in most cases. It doesn't sound like you have this. For a mediation to be significant, your IV needs to "cause" a mediator to such an extent that the mediator's effect on the DV can be at least partially attributed to your IV.
Here is some good basic information on mediation:
http://davidakenny.net/cm/mediate.htm