

Multiple mediation, FIML, and bootstrap 

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Hello. I'm trying to run a multiple mediation model (1 IV, 2 mediators, 2 outcomes) with FIML, and with bootstrapped SEs and biascorrected CIs. A main goal is to examine specific indirect effects. Assuming I have read in the data correctly, etc., does the following segment of code look correct? analysis: type = general missing H1; estimator = ML; iterations = 50000; bootstrap = 10000; model: med1 on indvar; med2 on indvar; outcome1 on med1 med2 indvar; outcome2 on med1 med2 indvar; med1 with med2; outcome1 with outcome2; model indirect: outcome1 IND med1 indvar; outcome1 IND med2 indvar; outcome2 IND med1 indvar; outcome2 IND med2 indvar; output: sampstat standardized cinterval(bcbootstrap); Also, under the "Model results" section, I assume "Estimates" are unstandardized and "Std" are standardized, but I get equivalent "Estimates" and "Std" results. So I guess "StdYX" are standardized? Under the "total, total direct, specific indirect, and direct effects" section, again I assume "Estimates" are unstandardized and "StdYX" are standardized? Thanks very much for your help. 


It's hard to look at code and say if it looks okay. The best way to check is to run it and see if you get any errors, and if not, to check the results to see if you get what you expect. Estimates are not standardized. Both Std and StdYX are standardized. Std uses the variances of latent variables. Because you have none, they are therefore the same as the raw coefficients. Same for indirect effects. See the user's guide for more information. 


I didn't get any errors, but things are not always as they seem. I think I have done enough now, though, to be confident in that part of the code. And thank you for clarifying about the raw and standardized estimates. What I am trying to do now is a direct comparison of the two mediators in the model, to determine whether one is a stronger predictor. So, I would like to create a contrast of the two specific indirect effects (specific indirect effect for mediator 1 minus specific indirect effect for mediator 2) and to bootstrap a CI for it. Can you provide some guidance on how to code this in the program above? Thanks very much. 


You can use MODEL CONSTRAINT to create indirect effects and test their equality. Example 3.10 shows an example of how MODEL CONSTRAINT is used. See also the full description of MODEL CONSTRAINT in the user's guide. You can then use the CINTERVAL option to obtain bootstrapped confidence intervals. See the description of the CINTERVAL option in the user's guide. 


Hi Linda and Bengt, I have run a model with multiple mediators using FIML, have bootstrapped the model, have asked for indirect effects and confidence intervals. For the significant specific effects (i.e., +/ 1.96) the confidence intervals (95%) do not pass through zero, as I would expect. However, I have three effects that are not significant according to the est/S.E (zscore), yet the confidence intervals do not pass through zero. For example, for an a>b>c relationship I have a unstandardised bcoeff of .13, a S.E. of .08, and a zscore of 1.76. Yet the lower CI is .32, and the upper CI is .04. Do you know what is going on here, and what I can conclude? Is this a significant indirect effect or not? Many thanks for your help in advance. 


The zscore should be compared to a symmetric confidence interval. The fact that the bootstrapped confidence interval does not behave like the symmetric confidence interval suggests you should use the bootstrapped confidence interval. 

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