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From |
Barbro Widerstedt <appoloniak@gmail.com> |

To |
statalist@hsphsun2.harvard.edu |

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

Date |
Tue, 22 Nov 2011 09:16:23 +0100 |

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 >> >> /Barbro Widerstedt > > > * > * 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/ > * * 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:*Nick Cox <njcoxstata@gmail.com>

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

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