Specific indirect effects using multigroup permutation
Posted: Mon Aug 26, 2019 6:05 am
I am estimating the below attached model. The model includes a predictor variable, two "mediating variables" (though the data are cross-sectional), and six outcome variables. I would like to use the multi-group permutation approach to compare the five countries in my data-set on this model. I am particularly interested in comparing the direct paths from PPG to the two "mediators" and to each the outcome variables, as well as the specific indirect effects of PPG on the outcomes via BPNS and PPG on the outcomes via BPNF (separately). The results of the permutation procedure, deliver just a single "indirect effect" which makes sense, but I'm wondering if there is a way to extrct the "per mediator" indirect effects without having to run separate path models (one with BPNS and one with BPNF).
For example, if I run this model using the Bootstrapping option in Calculate, I get specific indirect effects for PPG-BPNS-outcomes and PPG-BPNF-outcomes, but I don't get specific indirect effects when using PLS-MGA or permutation. The group-level specific indirect effects are easy enough to calculate manually (or just take from a bootstrap analysis, not MGA or permutation), but that will forestall the comparison of the specific indirect effects across groups.
Elaborating on the above, I have run a Bootstrap of the full model (included in the illustration below), for just one country. From this analysis, I can extract a mean specific indirect effect of BPNF (PPG -> BPNF -> A) of -0.231 (SD= 0.05, p < .001). Then, I conduct a PLS-MGA using ONLY BPNF as the sole mediator (so that I can isolate the specific indirect effect of BPNF separate from BPNS), comparing this country with another country. From the PLS-MGA results, I take the country specific indirect effect (PPG -> A), which is: original indirect effect of -0.326 (SD = 0.06, P < .001) (mean across bootstraps = -0.308).
Taken together, the results suggest that I cannot take the specific indirect effects from models that separately include either BPNF or BPNS as a mediator, because the indirect effects aren't the same in a model with one mediator, to the model which includes both.
It would be great if someone could provide some advice here.
For example, if I run this model using the Bootstrapping option in Calculate, I get specific indirect effects for PPG-BPNS-outcomes and PPG-BPNF-outcomes, but I don't get specific indirect effects when using PLS-MGA or permutation. The group-level specific indirect effects are easy enough to calculate manually (or just take from a bootstrap analysis, not MGA or permutation), but that will forestall the comparison of the specific indirect effects across groups.
Elaborating on the above, I have run a Bootstrap of the full model (included in the illustration below), for just one country. From this analysis, I can extract a mean specific indirect effect of BPNF (PPG -> BPNF -> A) of -0.231 (SD= 0.05, p < .001). Then, I conduct a PLS-MGA using ONLY BPNF as the sole mediator (so that I can isolate the specific indirect effect of BPNF separate from BPNS), comparing this country with another country. From the PLS-MGA results, I take the country specific indirect effect (PPG -> A), which is: original indirect effect of -0.326 (SD = 0.06, P < .001) (mean across bootstraps = -0.308).
Taken together, the results suggest that I cannot take the specific indirect effects from models that separately include either BPNF or BPNS as a mediator, because the indirect effects aren't the same in a model with one mediator, to the model which includes both.
It would be great if someone could provide some advice here.