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RE: st: means compairison with weights and unequal variance


From   Nick Cox <n.j.cox@durham.ac.uk>
To   "'statalist@hsphsun2.harvard.edu'" <statalist@hsphsun2.harvard.edu>
Subject   RE: st: means compairison with weights and unequal variance
Date   Mon, 21 Nov 2011 10:31:36 +0000

Describing -local-s as variables is very likely to confuse in any Stata context. Similarly, you are free to think of Stata variables as vectors or columns privately, but Stata has a separate syntax for vectors strict sense. 

See my postings earlier in the thread. 

Nick 
n.j.cox@durham.ac.uk 

Barbro Widerstedt

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.

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
>

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