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what is -group()-? [Was: Re: st: Extract and save R2 as a variable from looping regressions]


From   Nick Cox <njcoxstata@gmail.com>
To   "statalist@hsphsun2.harvard.edu" <statalist@hsphsun2.harvard.edu>
Subject   what is -group()-? [Was: Re: st: Extract and save R2 as a variable from looping regressions]
Date   Thu, 24 Oct 2013 18:50:51 +0100

I needed to unearth some old manuals to be sure of this.

-group()- is now, and long has been, an undocumented function.

In ancient manuals, -group()- is explained as producing as nearly
equal-sized subsamples as possible. Its operation is seen at its
simplest like this:

. clear

. set obs 10
obs was 0, now 10

. gen group = group(2)

. l

     +-------+
     | group |
     |-------|
  1. |     1 |
  2. |     1 |
  3. |     1 |
  4. |     1 |
  5. |     1 |
     |-------|
  6. |     2 |
  7. |     2 |
  8. |     2 |
  9. |     2 |
 10. |     2 |
     +-------+

What it does when given e.g. a variable as argument was not
documented, so far as I can find out.

Nick


njcoxstata@gmail.com


On 1 October 2013 18:23, Robert Picard <picard@netbox.com> wrote:
>>  . gen gr_compIDmofd=group (companyID mofd)
>>
>> is illegal. You must have used -egen-
>
> Curiously, using -group()- with -generate- does not throw an error but
> I have no idea what it's supposed to do. I can't find anything in the
> help files about it either.
>
> sysuse auto
> gen g = group(turn)
>
> Does anyone know what it does?
>
> Robert
>
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