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Estee posted on Friday, October 04, 2013 - 8:57 am
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Hi, I am using mplus to run an analyses of a multiple mediator model (3 mediators). I created pairwise contrast such as below: Con1 = a1b1 - a2b2 Con2 = a1b1 - a3b3 Con 3 = a2b2 - a3b3 In the output, the estimates of these contrasts appear at the part of "Model Results" which is before "Standardized Model Results". So, I am wondering the estimates for these contrasts in the output is standardized or unstandardized? Is it possible to get standardized estimates for these contrasts? |
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If you created these in MODEL CONSTRAINT, they are unstandardized. |
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Estee posted on Friday, October 04, 2013 - 10:56 am
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Dear Dr. Linda, so, how can I get standardized estimates for these contrasts? Thank you very much for your prompt reply. |
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You would need to do this using model constraint. You would need to specify the standard deviations for con1, con2, and con3 and specify the standardized coefficients. |
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Estee posted on Friday, October 04, 2013 - 3:42 pm
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How to specify standard deviations and standardized coefficients? where to get these values? Example of syntax? TQ. |
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First you have to decide if you want contrasts based on standardized coefficients or if you want a standardized contrast (divided by its SD). Let's assume the former (the latter is automatically obtained as the z score in Model Constraint). You are showing formulas with a*b which implies that you want to standardize a*b wrt the SDs of X and Y in the X->M->Y chain. So you need to be able to express the variance of Y from your model parameters. You do this by labeling parameters in the Model command, and then use those labels to computed the Y variance in Model Constraint. The X variance is obtained simply by the Model command labeling: x (xvar); |
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Hello, We are hoping to contrast several sets of indirect pathways in a SEM with one predictor, 4 mediators, and 3 outcomes. There are two mediators at time 1 and then the same mediators at time 2. We have calculated the indirect effect of the X --> MedA at time 1 --> MedA at time 2 --> Y using the VIA command, but we are unsure how to compare this to the indirect effect of X --> MedB at time 1 --> MedB at time 2 --> Y, since there are two mediators. Thank you so much for your help. |
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You can express the two indirect effects in Model Constraint and also their difference. It will then be tested for equality. |
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Bengt, Thank you for your response. I apologize for not being clear above and I have a quick follow-up question. For a model with one mediator (e.g. X --> M --> Y), you would specify the indirect effect as shown below: MODEL: y ON m (b1); m ON x (a1); MODEL CONSTRAINT: NEW(a1b1); a1b1=a1*b1 For a model with two mediators, how would you express this indirect effect? Is this correct for model X--> M --> L --> Y? MODEL: y ON l (c1); l ON m (b1); m ON x (a1); MODEL CONSTRAINT: NEW(a1b1c1); a1b1c1=a1*b1*c1 Or are the indirect effects not calculated with the product of the betas of the pathways when more than one mediator is included? Thanks again for your help! Jenna |
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Your formula a1*b1*c1 is correct for continuous M and Y. |
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