Re: contradictory results

Posted by Maguin, Eugene on Jan 19, 2007; 9:26pm
URL: http://spssx-discussion.165.s1.nabble.com/Re-contradictory-results-tp1073341p1073342.html

Mike,

So it sounds like you ran two different analyses. One was a repeated
measures ANOVA. The means for that analysis were as follows.

GROUP    pre   post  delta
Control  18.5  24.5   6.0
Tx       17.5  27     9.5

You had a significant interaction effect. If you look at the pre-post delta,
the Tx group has a bigger increment. But the control-tx difference at post
is only 2.5. However, if the control-tx difference at pretest had been 0.0
instead of 1.0, the post group difference would have been 3.5. So, I think
that one explanation is that the possibly nonsignificant difference at
pretest may be the cause of the nonsiginificant group difference at post.
Other things also matter, like within group SD at post. What I think you
should do is to use the emmeans subcommand and compare pre-post for control
and pre-post for tx. You'll have one of two outcomes: both significant or tx
significant but not control. The problem you have is that while both gained,
tx gained more.

It sounds like your second analysis was the regression equivalent of ANCOVA.
While there is no necessary reason for the two analyses to give the same
results, I think you made an interpretation error. The interaction that you
entered sounds like it must have been group by pretest. If so, then you are
testing whether the within group regression slopes are the same between the
two groups, which you should fervently hope that they are. You should be
looking at the group effect.

Gene Maguin