In sci.stat.edu Mike Tonkovich <[EMAIL PROTECTED]> wrote:
: Was hoping someone might be able to confirm that my approach for comparing 2
: slopes was correct.
: I ran an analysis of covariance using PROC GLM (in SAS) with an interaction
: statement. My understanding was that a nonsignificant interaction term
: meant that the slopes were the same, and vice versa for a significant
: interaction term. Is this correct and is this the best way to approach this
: problem with SAS? Any help would certainly be apprectiated.
Like any hypothesis testing situation, a non-significant interaction term
means that you failed to reject the null hypothesis, in this case, that
that the slopes are parallel. It's important to understand why this isn't
the same as saying the slopes are the same.
You have to be very careful about how to interpret this test. Depending
on a number of things, including the joint distributions of the main
effects involved, the significance test for the interaction term can
be associated with relatively low power.
Mike Babyak
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