RE: st: RE: tab

[David Kantor <dkantor@jhu.edu>]

-,subpop()- is like an -if-, or an additional -if- if there already is one; 
it limits the set of observations that are tabulated.  But the set of 
values appearing in the table are not limited by the subpop variable, and 
so, some cells can have zeroes.  (When applied to two-way tabulations, 
whole rows or columns might have all zeroes.)


I don't think you can successfully combine -,subpop()- with -bysort-, 
because the limiting subpopulation would need to be different for each 
value of the by-variable.

But you can do a separate -tab ..., subpop()- for each value of the 
categorical variable that you would normally think of as your bysort 
variable.  If this is done to one-way tabulations, then each instance is 
just like a column (or row) of a two-way tabulation, complete with cells 
having zeroes.

Thus, with the auto data, if all you want is the table for where foreign 
==1, you can do...

 tab rep78, subpop(foreign)

(This takes a shortcut, using the fact that foreign is coded 0 and 1.  More 
generally, you would do something such as...
 gen byte foreign1 = foreign==1 // or "Foreign":origin
 tab rep78, subpop(foreign1)

... and similarly for any other value corresponding to a desired limiting 
population.)

But in this case, if you were to do...
 gen byte foreign0 = foreign==0 // or "Domestic":origin
 tab rep78, subpop(foreign0)
you would get the same result as...
 tab rep78 if foreign==0
because there are no rep78 values beyond those seen within the set where 
foreign==0.


My comment about extending the dataset applies only if there are values of 
the tabulated variable that exist nowhere in the dataset.

I hope this is useful (and correct).
-- David

At 04:01 PM 12/11/2003 +0000, you wrote:

> Thanks for the reference to -, subpop()- which
> somehow has never registered with me; indeed
> I don't think I ever understood it.
>
> Here is the sort of example I had in mind. Some
> people would like explicit zeros in the case
> of rep78 == 1, 2 & foreign == 1. Can -, subpop()-
> help here, directly, without changing the dataset?
>
> (This was one motivation for writing -tabcount-,
> now on SSC. I thought that such tables should be
> possible on the fly, so far as the user is
> concerned.)
>
> . bysort foreign : tab rep78
> [...]
David Kantor
Institute for Policy Studies
Johns Hopkins University
dkantor@jhu.edu
410-516-5404

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