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Re: st: geometric means


From   Nick Cox <njcoxstata@gmail.com>
To   "statalist@hsphsun2.harvard.edu" <statalist@hsphsun2.harvard.edu>
Subject   Re: st: geometric means
Date   Wed, 12 Jun 2013 08:34:34 +0100

There's an -egen- function -gmean()- in -egenmore- (SSC), but the
problem  yields also to elementary mathematics and official Stata:

sysuse auto
egen gmean = mean(ln(mpg)), by(rep78)
replace gmean = exp(gmean)

Note that you can't take this further and go

egen gmean2 = exp(mean(ln(mpg))), by(rep78)

because -egen- expects to see an -egen- function on the right-hand
size and -exp()- is not an -egen- function. Why then is the first code
OK? Because the -egen- function -mean()- can work on an expression and
a call to a Stata function is fine.

The overloading of the word "function" needs a little care here.
-egen- functions are those documented under -egen-. Stata functions
are documented under -functions-. There is sometimes scope to use
Stata functions _inside_ calls to -egen- functions, as here. -egen-
functions cannot be used outside -egen- in any circumstances.

Nick
njcoxstata@gmail.com


On 11 June 2013 12:42, Nick Cox <njcoxstata@gmail.com> wrote:
> Check out -egen-.
>
> Nick
>
>
> On 11 Jun 2013, at 10:35, Abu Camara <abucamara@gmail.com> wrote:
>
>> Hi Stata list,
>>
>> I want to compute geometric means by year (time series)  for a list of
>> countries with Stata's "ameans" command.
>> BY and OVER are not supported by ameans.
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