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

To |
statalist@hsphsun2.harvard.edu |

Subject |
Re: st: means compairison with weights and unequal variance |

Date |
Tue, 22 Nov 2011 08:40:22 +0000 |

See also the suggestions made earlier in the thread at http://stata.com/statalist/archive/2011-11/msg01010.html Nick On Tue, Nov 22, 2011 at 8:16 AM, Barbro Widerstedt <appoloniak@gmail.com> wrote: > Now I feel a bit stupid -- of course. It is what I do for other > outcomes, and the strategy should be as valid in this case... I'll > have a look and see if they give me the same conclusion at p<0.05 > > > > On Mon, Nov 21, 2011 at 7:45 PM, Ariel Linden, DrPH > <ariel.linden@gmail.com> wrote: >> Why not simply use -regress- and the weight generated in -cem- (cem_weights) >> as the aweight with robust se? This is the approach suggested by the >> authors. See: >> >> Stefano M. Iacus, Gary King, and Giuseppe Porro, "Matching for Causal >> Inference Without Balance Checking", copy at >> <http://gking.harvard.edu/files/abs/cem-abs.shtml> >> >> Ariel >> >> >> >> Date: Sun, 20 Nov 2011 21:40:40 -0800 >> From: John Luke Gallup <jlgallup@pdx.edu> >> Subject: Re: st: means compairison with weights and unequal variance >> >> Barbro, >> >> A simple alternative is to calculate the means and standard deviations for >> each group using -summarize- with weights, and then run -ttesti ..., >> unequal-: >> >> sysuse auto, clear >> >> sum mpg if foreign [aw=weight] >> local N1 = r(N) >> local av1 = r(mean) >> local sd1 = r(sd) >> >> sum mpg if !foreign [aw=weight] >> local N2 = r(N) >> local av2 = r(mean) >> local sd2 = r(sd) >> >> ttesti `N1' `av1' `sd1' `N2' `av2' `sd2', unequal >> >> John >> >> John Luke Gallup >> Department of Economics >> Portland State University >> >> On Nov 20, 2011, at 2:13 AM, appoloniak wrote: >> >>> Hello statslisters, >>> >>> [caveat: sorry if this is a FAQ, but sometimes my imagination in >>> creating queries for use in the archives gives me nothing... and it is >>> more of a statistics that a Stata question, so please don't hit me too >>> hard ... ] >>> >>> I have a dataset where I try to compare the means of a variable >>> between two groups (treated and untreated). >>> The data set used is a sample, drawn from the superpopulation by the >>> ado-package cem (Iaucus et al Coarsened enhanced mathing), and >>> subsequent estimations should be weighted. >>> >>> This means that a standard t-test cannot be used, and I searched a bit >>> and found that <oneway> is an alternative with weighted data. However, >>> the groups have unequal variance which is a problem for <oneway> (at >>> least I think so, I know ANOVA mainly by name ...). I read one entry >>> that suggests that oneway is robust to groupwise unequal variance if >>> groupsize does not vary too much, but in my case they do (min >>> groupsize=2 max groupsize=1273) >>> >>> ttest <outcome>, by(treatvar) unequal -> t = -2.43 >>> oneway <outcome> <treatvar> [aweight=cem_weight] -> F=4.06 >>> >>> both bartlett's test for equality of variance, a standard sdtest , >>> and robvar suggest that I have unequal variance between groups. >>> >>> Suggestion on alternatives would be greatly appreciated * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

**Follow-Ups**:**Re: st: means compairison with weights and unequal variance***From:*Barbro Widerstedt <appoloniak@gmail.com>

**References**:**Re: st: means compairison with weights and unequal variance***From:*"Ariel Linden, DrPH" <ariel.linden@gmail.com>

**Re: st: means compairison with weights and unequal variance***From:*Barbro Widerstedt <appoloniak@gmail.com>

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