Re: st: Testing a curvilinear mediator

["Dirk Deichmann" <dirk@dirkdeichmann.de>]

Hey everyone,

Does anyone know how to handle and interpret an inverted u-shaped mediator in a logistic regression? Or do you maybe have some literature tips?

Many thanks again,

Dirk

----- original message --------

Subject: st: Testing a curvilinear mediator
Sent: Wed, 16 Mar 2011
From: Dirk Deichmann<dirk@dirkdeichmann.de>

> Hi group,
> 
> I would like to test a model with a mediating variable which by itself has a
> curvilinear relationship with the DV. IV and DV are binary variables whereas
> M is continuous.
> 
> If such an analysis of a mediator, which is curvilinear, is possible, how
> can I perform it? Are there some causal steps I need to go through? 
> 
> I thought that maybe a moderated mediation would work except that the
> moderator is not a different variable but the same as the mediator.
> 
> I started with the four step procedure and added the squared term in step 3
> next to the linear term, and the IV (see steps and findings below). However,
> I have read that for moderated mediation it is advised to also include an
> interaction of the moderator with the IV in the first step already. But this
> seems not really applicable in my case, or did I misunderstood something? 
> 
> Provided that I am doing something reasonable here, how do I actually
> interpret a curvilinear mediator? Is it correct to say that the mediator
> works best at intermediate levels and less at low and high levels if I find
> an inverted U-shaped relation? Also, how should I perform the Sobel test?
> Does this make sense at all if my mediator turns out to be inverted
> U-shaped?
> 
> Path	b	(s.e.)	sig.
> Step 1: IV > DV	0.63	(0.36)	^
> Step 2: IV > M	0.25	(0.14)	^
> Step 3: M > DV (controlling for IV and M squared)	1.84	(0.38)	***
> Step 3: M squared > DV (controlling for IV and M)	-0.50	(0.15)	***
> Step 4: IV > DV (controlling for M and M squared)	0.44	(0.38)	n.s.
> 
> Lot?s of questions- I hope that somebody knows something about this and
> would like to thank you in advance for your kind help and support. 
> 
> Best,
> 
> Dirk
> 
> 
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