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


From   Gwinyai Masukume <parturitions@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: Interpretation of Two-sample t test with equal variances?
Date   Wed, 20 Mar 2013 07:04:38 +0200

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

On 3/20/13, Richard Williams <richardwilliams.ndu@gmail.com> wrote:
> Your t-test seems to suggest that age is affected
> by mode of delivery, rather than mode of delivery
> is affected by age. It probably won't change
> things much but this makes more sense to me given your hypotheses:
>
> logit mode_delivery age
>
> At 11:08 PM 3/19/2013, Gwinyai Masukume wrote:
>>Dear Stata list,
>>
>>I would like to double check the interpretation and appropriateness of
>>the following statistical test I performed.
>>My alternate hypothesis is that, “There is a difference in the baby’s
>>mode of delivery depending on maternal age” And the null hypothesis is
>>that, “There is no difference in the baby’s mode of delivery depending
>>on maternal age”
>>Looking at the output ­ “Ha: diff != 0, Pr(|T| > |t|) = 0.4861”, I
>>fail to reject the null hypothesis and conclude that, “There is no
>>difference in the baby’s mode of delivery depending on maternal age”
>>
>>Is this a sound and appropriate interpretation?
>>
>>. *** Doing a T-test
>>. ttest age, by(mode_delivery)
>>
>>Two-sample t test with equal variances
>>------------------------------------------------------------------------------
>>    Group |     Obs        Mean    Std.
>> Err.   Std. Dev.   [95% Conf. Interval]
>>---------+--------------------------------------------------------------------
>>  Vaginal
>> |     166    28.83072    .4696729    6.051313    27.90338    29.75807
>>C/sectio |      84    29.39524    .6579862    6.030543    28.08653
>> 30.70395
>>---------+--------------------------------------------------------------------
>>combined |     250     29.0204    .3818851    6.038134    28.26826
>> 29.77254
>>---------+--------------------------------------------------------------------
>>     diff
>> |           -.5645152    .8093331               -2.158558    1.029528
>>------------------------------------------------------------------------------
>>     diff = mean(Vaginal) -
>> mean(C/sectio)                         t =  -0.6975
>>Ho: diff = 0                                     degrees of freedom =
>> 248
>>
>>     Ha: diff < 0                 Ha: diff != 0                 Ha: diff >
>> 0
>>  Pr(T < t) = 0.2431         Pr(|T| > |t|) =
>> 0.4861          Pr(T > t) = 0.7569
>>
>>With kind regards,
>>Gwinyai
>>
>>*
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>
> -------------------------------------------
> Richard Williams, Notre Dame Dept of Sociology
> OFFICE: (574)631-6668, (574)631-6463
> HOME:   (574)289-5227
> EMAIL:  Richard.A.Williams.5@ND.Edu
> WWW:    http://www.nd.edu/~rwilliam
>
>
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