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Re: 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   Re: what is -group()-? [Was: Re: st: Extract and save R2 as a variable from looping regressions]
Date   Thu, 24 Oct 2013 19:03:05 +0100

I overlooked an earlier reply by Phil Schumm, which shows that I gave
a better answer in 2007:

http://www.stata.com/statalist/archive/2007-03/msg00406.html
Nick
njcoxstata@gmail.com


On 24 October 2013 18:50, Nick Cox <njcoxstata@gmail.com> wrote:
> 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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