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Topic: ANOVA Interactions vs. Simple Z tests
Replies: 2   Last Post: Jul 29, 1996 9:26 AM

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Marc Mednick

Posts: 2
Registered: 12/18/04
ANOVA Interactions vs. Simple Z tests
Posted: Jul 26, 1996 3:23 PM
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I recently used ANOVA to see if there was a significant interaction in a
before-and-after treatment-vs-control experiment. (Two levels within each
of two factors, with a single dependent variable.)

However, upon presenting my results it was suggested that instead of using
ANOVA, I should use a simple Z-test, as follows:

Difference of differences = (t2-t1) - (c2-c1)

where t2 = "After" treatment mean
t1 = "Before" treatment mean
c2 = "After" control mean
c1 = "Before" control mean

and the standard error of this mean would be equal to the square root of
the four individual variances summed.

Something about using this Z-test procedure instead of ANOVA doesn't sit
well with me, though I'm having trouble pinpointing why. It seems like
there's information not being captured, but I'm having difficulty
explaining what exactly is wrong, if anything, with Z-tests instead of

I'd appreciate any insight into this issue.

Thank you.


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