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Hi, I am performing a causal mediation analysis with binary exposure x, binary mediator m and binary outcome y, but am getting unexpected estimates of the direct and indirect effect. Model input: m on x; y on x; y on m; model indirect: y ind m x ; The model coefficients show a strong independent positive association between x and m, m and x, and x and y, so I would expect some direct and some indirect effects. But the estimate of natural DE and IE show there is no indirect effect at all I find this strange given that there is a strong association between X and M, and M and Y. I am estimating the model incorrectly? Many thanks, Katy 


We just responded to you on Support where the issue seems to be that the counterfactual causal effects are conditioned on covariates = 0. The value 0 may not be the substantively relevant value to condition on. When these covariates were centered so that their means are 0 you got larger effects. 


Hi Bengt, Thanks for the 2014 paper “Causal Effects in Mediation Modeling: An Introduction With Applications to Latent Variables” and in bringing latent variables into the counterfactual models. I have a question regarding Nominal mediators. I have a binary outcome and a nominal mediator with three categories. Do I have to use the known class method as is done in your previous working paper? Or is there an easier way of doing it in mplus version 7.3? your guidance on this is appreciated. Thanks, Selahadin 


No easier way in 7.3; our new language doesn't cover that case. So, yes, take the Knownclass approach. And if you get a nice application written up, please send to me. 


Thank you Bengt. Selahadin 


Would it be possible to direct me to a source/paper that would explain why the PNDE is not significant in my model but the path coefficients are? Or can I explain it as the PNDE is the pure effect from IV to DV and the path coefficients are also taking into account other variables in the model, meaning they are eating up the variance potentially allowing for that path from IV to DV to come through? Thank you, Kathleen 


Are both M and Y continuous? Are you using ML? ML with bootstrapping? Or Bayes? 


Hi, M is continuous but Y is binary zeroinflated. I'm using WLSMV. Thanks 


Binary, zeroinflated sounds strange; I am not sure that makes sense  just do binary. If this doesn't help; please send your output to Support along with your license number. 


Hi again, Thank you. I have chosen WLSMV based on past discussions, as my dataset as way more zeros than 1s. But could that be affecting the PNDEs? I didn't find it appropriate to use ML given the zeroinflation. I was just thinking that the path coefficients are derived by accounting for everything in the model, while the PNDE is only the effect from IV to DV, if I understand correctly. So I was thinking the path coefficients are significant because there is variance accounted for by other variables which is not the case for the PNDEs? Thanks! 


I would need to see the output to say what is going on. 

Amelia Rock posted on Wednesday, August 07, 2019  1:02 pm



Hello, I am conducting mediation analysis with continuous Xs, continuous and ordinal Ms, and a binary Y. I am trying to understand whether I can use the counterfactuallyestimated indirect effects I got when I ran the model without specifying the two exposure values to be compared as one must do when the exposure is continuous. The user guide says the default is one for the first value and zero for the second value, but I do not know whether this makes sense for my exposures (observed means of scales measuring experiences of stigma). Is there guidance for choosing the two values to be compared (understanding of course that subject area knowledge should be applied as well)? 


The scale of X is important. Standardize X so that 0 is the mean and 1 is one SD above the mean. Otherwise your results are meaningless. 

Amelia Rock posted on Friday, August 09, 2019  11:14 am



Makes perfect sense, many thanks. 

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