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st: Calculation of total effects and the significance test of indirect effect


From   "Michael Cha" <mikecha0@lycos.com>
To   statalist@hsphsun2.harvard.edu
Subject   st: Calculation of total effects and the significance test of indirect effect
Date   Thu, 30 Sep 2004 02:05:14 -0500

Dear listers,

I am writing to ask a few questions regarding the calculation of total effect = direct effect + indirect effect. 

Suppose I am interested in the relationship between X and Y and Z.  I also have demographic variables to consider such age and sex. 

1) My first question is, 
If I want to calculate the total effect of X on Y, do I have to control for Age and Sex or not? 
In other words, is Rxy the total effect or Rxy.age,sex the total effect? 

2) My second question is, 
As far as I know, the SEM package softwares (e.g. Lisrel or Amos) automatically calculate the direct and indirect effect of included components. 
Unfortunately, I now do not use such softwares but just SPSS, and need to calculate the total, direct and indirect effect between X Z and Y. 
Then, is there anyway to test the significance of indirect effect of path coefficients a*b (which are multiplied indirect effects of the path from X to Z(a) and Z to Y(b)?

Thank you very much for your attention and help in advance and sorry if these are too basic questions to look at. 

I will appreciate it much more if you can suggest any reading materials to grasp these topics more clearly. 

Regards,
Mike 
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