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Re: st: Interpretation of Two-sample t test with equal variances?


From   David Hoaglin <dchoaglin@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: Interpretation of Two-sample t test with equal variances?
Date   Wed, 20 Mar 2013 12:43:50 -0400

Despite the tolerance of Student's t for departures from assumptions,
comparing the (mean) ages in the two outcome groups is not the right
approach.  For initial exploration, side-by-side boxplots of age (by
mode_delivery) would give an indication of skewness, as well as
presence of unusually low or high ages.  Apart from giving some points
undue influence, skewness in the distribution of a predictor (per se)
is often not important.  If the predictor could be transformed, to
straighten a nonlinear relation with the outcome, the transformation
might (as a by-product) reduce or remove the skewness.

David Hoaglin

On Wed, Mar 20, 2013 at 12:16 PM, Gwinyai Masukume
<parturitions@gmail.com> wrote:
> Thank you so much everyone. Appreciated.
>
> David - it was indeed a very helpful discussion.
> Nick - indeed those are means of maternal age. you are significant.
> yes, the mother's ages are skewed. what do you mean by student's t
> test works well even if you lie to it?
> Carlo - it seems all the relevant independent variables have not been
> included, the very low pseudo r2 is bizarre to me.
>
> Thanks again.
> Gwinyai
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