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From |
Nick Cox <njcoxstata@gmail.com> |

To |
statalist@hsphsun2.harvard.edu |

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

Date |
Wed, 20 Mar 2013 14:33:39 +0000 |

In much the same spirit as earlier suggestions: The mean ages given were 28.8 and 29.4 (presumably years) for the two classes. That sounds like a difference without clinical significance, although I am no clinician, not a woman, and not even significant. However, it is also likely that the means are hiding important details in the distributions. For example, I would expect skewed distributions for mothers' ages -- and the skewness I might guess to differ between the two modes of delivery. General knowledge underlines a range from <<20 to >50 years. Although I have much faith that Student's t test works well even if you lie to it, skewness sounds like an area for investigation. My gut instinct is that turning the problem round to make it a logit regression on age makes much more sense. I would use a fractional polynomial or cubic spline in age and always plot some smooth summary of one or other fraction (e.g. fraction C or fraction V) versus age. Nick On Wed, Mar 20, 2013 at 2:02 PM, David Hoaglin <dchoaglin@gmail.com> wrote: > 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 >> ------------------------------------------------------------------------------- >> * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/faqs/resources/statalist-faq/ * http://www.ats.ucla.edu/stat/stata/

**Follow-Ups**:**Re: st: Interpretation of Two-sample t test with equal variances?***From:*"JVerkuilen (Gmail)" <jvverkuilen@gmail.com>

**References**:**st: Interpretation of Two-sample t test with equal variances?***From:*Gwinyai Masukume <parturitions@gmail.com>

**Re: st: Interpretation of Two-sample t test with equal variances?***From:*Richard Williams <richardwilliams.ndu@gmail.com>

**Re: st: Interpretation of Two-sample t test with equal variances?***From:*Gwinyai Masukume <parturitions@gmail.com>

**Re: st: Interpretation of Two-sample t test with equal variances?***From:*David Hoaglin <dchoaglin@gmail.com>

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