I have performed a mediation analysis. I have an independent variable T, a mediator M, and outcome Y. (All 3 variables are binary, and I use logit.) (While I used Stata
medeff command to run the mediation analysis, this is NOT a question about
(I make reference to the standard mediation triangle, where path a connects T and M, path b connects M and Y, and path c' connects T and Y.)
Equation 1 is simply regressing M on T and some control variables. (this gives an estimate for path a) Equation 2 regresses Y on M + T +control variables. (this gives estimates for b and c')
What I find is the following:
I get a significant negative association between T and M. (path a) I get a significant positive association between T and Y. (path c') I get a "non-significant" association between M and Y (path b)
Equation 1 M Coef. Std. Err. z P>z [95% Conf. Interval] T -3.458955 .1044635 -33.11 0.000 -3.6637 -3.25421 Equation 2 Y Coef. Std. Err. z P>z [95% Conf. Interval] T .6081007 .0949606 6.40 0.000 .4219813 .7942201 M -.1496489 .0989504 -1.51 0.130 -.3435881 .0442904
So, path a is significant and negative, path b is not significant, and path c' is positive and significant. Based on that, from what I understand from Baron and Kenny 4 steps, I would conclude that there is no mediation because path b is not significant. However,
medeff produces the following results:
========================================================== Effect Mean [95% Conf. Interval] ACME1 .0189176 -.005018 .0438004 ACME0 .0160403 -.0041128 .0379756 Direct Effect 1 .1274808 .0903369 .1646811 Direct Effect 0 .1246035 .0863965 .1633032 Total Effect .1435211 .1134255 .1722113 % of Total via ACME1 .1313853 .1098513 .1667854 % of Total via ACME0 .1114022 .0931433 .141418 Average Mediation .017479 -.0045654 .0409513 Average Direct Effect .1260421 .088528 .1640466 % of Tot Eff mediated .1213937 .1014973 .1541017 ============================================================
medeff says there is 12% mediation by M (looking at the last row, % of Total Effect mediated.)
So, my questions are:
- Given the results from the two regression analysis above, should I ignore the
medeffresults and conclude that there is no mediation? Or is there actually mediation?
- Why if
medeffsays that there is 12% mediation, would the confidence interval for "average mediation" include 0, ie not significant?
- Is it enough to simply report the % of total effect mediated? or do I also have to report average mediation?