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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 |
Mon, 21 Nov 2011 10:39:18 +0100 |

local is a _variable_ (a scalar value), while - gen- generates a new column ( a vector of values) r(N) is the number of observations in your data set (=a number) , if you successfully - gen - you would violate the requirements of the command, which is for scalar values. /B On Mon, Nov 21, 2011 at 10:02 AM, David Ashcraft <ashcraftd@rocketmail.com> wrote: > John, > > I am trying to implement the code but replacing local with gen. Why I am not getting the same result, if I use gen rather than local? > > David > > > ----- Original Message ----- > From: John Luke Gallup <jlgallup@pdx.edu> > To: statalist@hsphsun2.harvard.edu > Cc: > Sent: Monday, November 21, 2011 8:40:40 AM > 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/ > > > * > * 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 <n.j.cox@durham.ac.uk>

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

**Re: st: means compairison with weights and unequal variance***From:*John Luke Gallup <jlgallup@pdx.edu>

**Re: st: means compairison with weights and unequal variance***From:*David Ashcraft <ashcraftd@rocketmail.com>

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