Date:         Sat, 14 Aug 2010 00:24:19 -0400
Reply-To:     R B <ryan.andrew.black@gmail.com>
Sender:       "SPSSX(r) Discussion" <SPSSX-L@LISTSERV.UGA.EDU>
From:         R B <ryan.andrew.black@gmail.com>
Subject:      Re: MIXED interaction with contrasts
In-Reply-To:  <201008101540.o7AAktwV000446@malibu.cc.uga.edu>
Content-Type: text/plain; charset=ISO-8859-1

Charla,

I realize this response is late, but I just came across this post. If
you treat "Week" as a categorical variable, then you should be able to
test if there is a difference in linear trends between Group 1 and
Group 3 by adding the following TEST statement:

  /TEST = 'Linear Trend Contrast 1 v 3' Group*Week  -3 -1  1  3

      0  0  0  0

      3  1 -1 -3

If you wanted to test if there is a difference in linear trends
between Group 2 and Groups 1 and 3, then you would add the following
TEST statement:

  /TEST = 'Linear Trend Contrast 2 v 1 and 3' Group*Week  1.5  0.5 -0.5 -1.5

             -3.0 -1.0  1.0  3.0

              1.5  0.5 -0.5 -1.5

These recommended TEST statements are untested, so you will need to
confirm that these are correct.

On another note, I suggest you consider accounting for the possible
residual covariance due to repeated measures by adding a REPEATED
statement (e.g., unstructured, autoregressive).

HTH,

Ryan

On Tue, Aug 10, 2010 at 11:40 AM, Charla Lopez <charla3@aol.com> wrote:
> Hi there-
> I would like to find out whether there is an interaction between treatment
> group and time, using a special contrast of treatment group.  In my example,
> treatment group has 3 levels, and time has 5 levels.  I am able to get the
> output for the custom hypotheses, but the interaction effects seem to be
> based on the default coding for treatment group.
>
> Here is my syntax:
>
> MIXED WEEKLYCIGS by GROUP
>  with  WEEK
>  /FIXED =
>  GROUP
>  WEEK
>  GROUP*WEEK
>  | SSTYPE (3)
> /test 'G1 v G3' group -1/2 0 1/2
> /test 'G2 v G1 and G3 ' GROUP -1/2 1 -1/2
>  /PRINT SOLUTION LMATRIX/METHOD = REML
>  /RANDOM INTERCEPT |SUBJECT (ID).
>
> Any advice?
>
> Charla
>
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