Use a permutation test
-------- Original Message --------
Subject: Help with ANCOVA
From: Graham Stirrat Clarke <[log in to unmask]>
Date: Tue, April 01, 2014 6:17 am
To: [log in to unmask]
Dear All,
A colleague has a before and after treatment dataset to analyse. He
has the pre-variable, the post-variable and a treatment variable. He
has used ANCOVA. However looking at his data both the pre-variable
and post-variable have a far from normal distribution (looks like an
exponential, with lots of 0 and 1 values dropping down smoothly to 12
– there are no negative values). I’m not convinced he can use
ANCOVA in these circumstances or am I wrong?
One thing I’ve noticed is that the difference between pre and post
values does follow an approximately normal distribution. Could we
analyse the differences instead? Would repeated measures ANOVA or one
of its derivatives be acceptable?
Any help or advice you could offer would be appreciated.
Thank you,
Graham
Rhif Elusen Gofrestredig 1141565 - Registered Charity No. 1141565
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