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Re: st: Mixed effects model for asymmetric data


From   "JVerkuilen (Gmail)" <jvverkuilen@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: Mixed effects model for asymmetric data
Date   Wed, 26 Sep 2012 13:23:47 -0400

On Wed, Sep 26, 2012 at 1:13 PM, Ana Beatriz FS
<nutrianabeatriz@gmail.com> wrote:
> Dear Statalisters,
>
> I was performing multilevel mixed-effects linear regression but I
> realized my data is not normally distributed. Is there an equivalent
> model for asymmetric data in stata?
>
> I've tried to transform my data but I could not accomplish because my
> data require different transformations in each point of follow-up. My
> sample is composed by pregnant women in the three trimesters of
> pregnancy, but values of my dependent variable in first trimester
> require log transformation for normalization, in the second, sqrt
> transformation and so on.

What are the measures? For instance, if they are counts, you might be
better off with -xtmepoisson-, which assumes that you have multilevel
Poisson distributed data. If the transformations are all fairly close,
you can probably get away with choosing one, so if the right answer is
sqrt and you log instead it won't be *that* far off. Also keep in mind
that the marginal distribution before conditioning on regressors can
be rather far from normal, so it's not really clear you need to do any
transformation. What do the residuals look like?


-- 
JVVerkuilen, PhD
jvverkuilen@gmail.com

"Out beyond ideas of wrong-doing and right-doing there is a field.
I'll meet you there. When the soul lies down in that grass the world
is too full to talk about." ---Rumi
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