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RE: st: Collapse with sum function and missing values


From   "Nick Cox" <n.j.cox@durham.ac.uk>
To   <statalist@hsphsun2.harvard.edu>
Subject   RE: st: Collapse with sum function and missing values
Date   Thu, 11 Feb 2010 14:51:08 -0000

The situations are quite distinct. 

1. There are no observations [in any sense] for which -rep78 == 0-, so
the core code of -summarize- never gets called up. 

2. When -summarize- reports "Obs 0" for -rep78 == .- this is to be
understood as a contraction of "Observations for which there are
non-missing values 0". There is hardly enough space in the Results
window even to abbreviate that. 

The point is explicit in the manual entry, but the definition of r(N) in
the online help could be tweaked to make it more obvious still. 

Nick 
n.j.cox@durham.ac.uk 

Martin Weiss

" Interestingly,

. su rep78 if rep78==0, detail"

But that is not the surprising part, since there are no "0" values for
"rep78". The weird thing is that 

*************
su rep78 if rep78==.,d
*************

returns "no observations" whereas -su rep78 if rep78==.- reports 0
observations.

Kit Baum

On Feb 11, 2010, at 2:33 AM, N.J. Cox wrote:

> - -summarize- 
> ===========
> 
> . sysuse auto
> (1978 Automobile Data)
> 
> . su rep78 if rep78 == .
> 
>    Variable |       Obs        Mean    Std. Dev.       Min        Max
> -
-------------+--------------------------------------------------------
>       rep78 |         0
> 
> . ret li
> 
> scalars:
>                  r(N) =  0
>              r(sum_w) =  0
>                r(sum) =  0


Interestingly,

. su rep78 if rep78==0, detail

                     Repair Record 1978
-------------------------------------------------------------
no observations


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