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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 10:02:44 -0400

Gwinyai,

In your first message you posed the question of whether the mode of
delivery depended on (or was related to) mother's age.  The logistic
regression is an appropriate way to approach that question.  The
output says that, in your data, the odds of a C/section increase with
mother's age, but the rate of increase does not differ significantly
from zero.  That is, the risk of a C/section is not related to
mother's age.

You may want to do a little diagnostic checking, to make sure that the
logit model is a satisfactory summary of your data.  You could split
the age range into intervals (with a reasonable total sample size in
each interval), and calculate the percentage of C/sections in each
category.  Does either group of mothers contain any unusually low or
unusually high ages?

I hope this discussion is helpful.

David Hoaglin

On Wed, Mar 20, 2013 at 1:04 AM, Gwinyai Masukume
<parturitions@gmail.com> wrote:
> Thank you Richard. Yes, I guess the t-test suggests the counter
> intuitive though it probably won’t change things much.
> How can I reverse the situation?
>
> I ran a logistic regression for binary outcomes as you suggested:
> Essentially no significance is shown?
>
> . logit mode_delivery age
>
> Iteration 0:   log likelihood = -159.58665
> Iteration 1:   log likelihood = -159.34203
> Iteration 2:   log likelihood = -159.34197
> Iteration 3:   log likelihood = -159.34197
>
> Logistic regression                               Number of obs   =        250
>                                                   LR chi2(1)      =       0.49
>                                                   Prob > chi2     =     0.4842
> Log likelihood = -159.34197                       Pseudo R2       =     0.0015
>
> -------------------------------------------------------------------------------
> mode_delivery |      Coef.   Std. Err.      z    P>|z|     [95% Conf. Interval]
> --------------+----------------------------------------------------------------
>           age |   .0155454   .0222368     0.70   0.485     -.028038    .0591288
>         _cons |  -1.133737   .6630978    -1.71   0.087    -2.433385    .1659111
> -------------------------------------------------------------------------------
>
> With thanks,
> Gwinyai

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