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Re: st: means compairison with weights and unequal variance


From   Nick Cox <njcoxstata@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: means compairison with weights and unequal variance
Date   Mon, 21 Nov 2011 09:33:20 +0000

This would work. But it is in no sense superior to John's original.

sysuse auto, clear
sum mpg if foreign [aw=weight]
gen N1 = r(N)
gen av1 = r(mean)
genl sd1 = r(sd)

sum mpg if !foreign [aw=weight]
genl N2 = r(N)
gen av2 = r(mean)
gen sd2 = r(sd)

ttesti `=N1[1]' `=av1[1]' `=sd1[1]'' `=N2[1]' `=av2'[1] `sd2[1]', unequal

The [1] subscripts are possibly superfluous; I have not tested this.


On Mon, Nov 21, 2011 at 9:09 AM, Nick Cox <njcoxstata@gmail.com> wrote:
> The syntax ` ' only de-references local macro names. Once you change
> the syntax as you did, the --ttesti- call reduces to
>
> ttesti, unequal
>
> which makes no sense. There is no reason to change it anyway.
>
> Nick
>
> On Mon, Nov 21, 2011 at 9:02 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
>>
>

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