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 learningmplus posted on Monday, November 28, 2011 - 9:07 pm
Hi!

I need some additional help following up on a previous post of mine.

I basically have 2 treatments. treatment a and treatment b. I am exploring moderators of treatment outcome.

I previously used


POST TX SCORE ON TYPE OF TREATMENT
POST TX SCORE ON PRE TX SCORE

and the MODERATOR (e.g. SEX as the GROUPING VARIABLE). I THEN TRIED PRODUCT TERMS BC I WAS TOLD THAT MULTIPLE GROUP REQUIRES LARGE SAMPLES.

Then my professor advised another student to create a difference score = pre-post and regress type of treatment into it if she wanted to get the interaction with treatment.


I am back at square 1 thinking that it is this difference score that I need to regress onto the product term between type of tx and the moderator. But I am not sure which way is the correct way. I am esentially looking at a 3 way interaction.

I even created files and looked at the means separately for each groups and the means I get by doing this in each group are consistent with the difference score approach and the findings make theoretical sense.

But when I use post on type of treatment controlling for pre and using multiple group it is not the same. Any help as to which is the correct way will be appreciate it. Thank you!!
 Linda K. Muthen posted on Tuesday, November 29, 2011 - 11:43 am
I would not use a difference score. I would use the following model:

post ON tx pre inter;

where the pretest is centered and inter is created in DEFINE as follows:

DEFINE:
pre = pre - a; where a is the mean of pre
inter = pre*tx;
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