Joint test of indirect effect x moderator

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No user picture. pascofearon Joined: 05/07/2013
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Hi Mike,
Hope you're well. I just wondered whether you had a suggestion for getting a single test of a moderator on an indirect (a*b) path using osmasem. Right now I can easily get the indirect effect itself, and estimates of it at high- and low- levels of the moderator, plus regressions of the respective paths on the moderator, but not a joint test of the indirect effect's variability as a function of the moderator... any tips?

Thanks!

Best wishes,

Pasco

Replied on Sun, 06/28/2020 - 20:32
Picture of user. Mike Cheung Joined: 10/08/2009

Hi Pasco,

I don't think that this is possible as a*b is not a parameter in the model. Moderators can only be used to predict parameters (a and b), not a*b.

Best,
Mike

Replied on Fri, 07/10/2020 - 10:48
Picture of user. AdminNeale Joined: 03/01/2013

Hi

It seems to me that one could moderate the entire indirect effect, ab. Usually we think about moderating individual paths, so a definition variable, say age, could affect parameter a, replacing it by a + beta_a\*age. Path b could be similarly moderated to become b + beta_b\*age. Equating the two moderations beta_a and beta_b would be somewhat equivalent to moderating the product ab by age. Note, however, the existence of other components to the mediation: (a+beta_a\*age)(b + beta_b\*age) is now the mediation effect (which will vary across values of age). The expansion is a\*b + beta_a\*age\*b + a*beta_b\*age + beta_a\*beta_b\*age\*age, so it's partly quadratic with respect to age whenever both betas are non-zero. Even if the betas are equated.

Replied on Fri, 07/10/2020 - 21:05
Picture of user. Mike Cheung Joined: 10/08/2009

Hi Michael,

Yes, the path coefficients a and b can be separately modeled by a moderator (m) as you suggested. But this model is different from a*b = beta0 + beta1*m. I have tried hard to figure out if it is possible to do the latter one.

Mike