Change the mediator distribution in Proc Causalmed

[Giampaolo]

Dear SAS users,

By default Proc Causalmed assigns a normal distribution to the mediator variable and an identity link. Is it possible to customize these parameters? The actual distribution of my mediator variable is closer to a gamma distribution. 

[SAS_Rob · Accepted solution]

PROC CAUSALMED assumes a normal distribution and the identity link function for a continuous mediator, and a binary distribution and the logit link function for a binary mediator.  Unfortunately there is no way currently to change this default behavior.

[Giampaolo]

Thank you.

[SteveDenham]

If your mediator looks like it may have a gamma distribution, consider a close approximation such as a lognormal.  To do this, take the natural log of the mediator variable - this will have a normal distribution of residuals in the log space and has a default identity link in the other PROCs that address alternatives to the normal distribution.  It then becomes even more important that you consider how you report the mediating effect.

SteveDenham

[Giampaolo]

Thank you for your suggestion!

Then how should I interpret the output given that the outcome is binary?

In summary I would have:

a binomial distribution and logit link for the outcome.

a normal distribution and identity link for a log transformed mediator.

The output would be given in Odds Ratios or excess relative risk, for  the direct,  indirect and total effects.

Can I still express  mediation per unit change of the mediator, rather than its log transformed value? If yes how?