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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 16:58:41 +0000 |

I wouldn't want to start with box plots here. For two groups, there is space for more detail than a box plot gives, except capriciously in so far as points are more than 1.5 IQR from the nearer quartile. I grew up, as it were, on box plots, including David's own writings 30 or more years ago, but I think they are oversold in total. I have often seen -- even in otherwise good textbooks -- people talking about t-tests or ANOVA and then giving box plots -- and not even commenting on the mismatch, i.e. that the box plots are showing medians etc., not means etc. A more directly relevant graph would, I suggest, be a dotplot (sensu -dotplot-) or even -stripplot- (SSC). You can show means on such plots. Nick On Wed, Mar 20, 2013 at 4:43 PM, David Hoaglin <dchoaglin@gmail.com> wrote: > 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 > * > * 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/ * * 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:*David Hoaglin <dchoaglin@gmail.com>

**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>

**R: st: Interpretation of Two-sample t test with equal variances?***From:*"Carlo Lazzaro" <carlo.lazzaro@tiscalinet.it>

**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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