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RE: st: Collapse & Missing Values


From   "Jian Zhang" <jzh@ucdavis.edu>
To   "Eric G. Wruck" <ewruck@econalytics.com>, statalist@hsphsun2.harvard.edu
Subject   RE: st: Collapse & Missing Values
Date   Wed, 28 Sep 2005 12:22:26 -0700 (PDT)

I run into the same problem.
What I know is the following: 
id var1 var2
1   12    .
1   14    2
2   16    . 
2   17    .
if you type 
gen var3=var1+var2,
stata will give you:

id var1 var2 var3
1   12    .   .
1   14    2   16
2   16    .   .
2   17    .   .

However, if you type 
sort id
by id: egen var3=sum(var2),
stata will give you
id var1 var2 var3
1   12    .   2
1   14    2   2
2   16    .   0
2   17    .   0

I wonder who can explain the way state deals with missing values? Is 
there any general rule for stata?
Thanks.

Jian


> I just learned, rather inconveniently, that collapse doesn't work the 
> way I'd like when encountering missing values.  Here's an example:
> . l
> 
>       +----------------------+
>       |        date   amount |
>       |----------------------|
>    1. | 10-Oct-1990      200 |
>    2. | 10-Oct-1990      -75 |
>    3. | 10-Oct-1990       64 |
>    4. | 11-Oct-1990        . |
>    5. | 12-Oct-1990      107 |
>       |----------------------|
>    6. | 12-Oct-1990        . |
>       +----------------------+
> 
> . collapse (sum) net_amt=amount, by(date)
> 
> . l
> 
>       +-----------------------+
>       |        date   net_amt |
>       |-----------------------|
>    1. | 10-Oct-1990       189 |
>    2. | 11-Oct-1990         0 |
>    3. | 12-Oct-1990       107 |
>       +-----------------------+
> 
> .
> The problem is for the single 11-Oct-1990 observation.  After 
> collapsing, the missing value becomes a zero; in this instance I 
> would have preferred it remain missing.  The 12-Oct-1990 treatment is 
> fine & what I expected.  I suppose I could delete observations before 
> performing the collapse but it would be better if there was some 
> other option.  Is there?
> 
> 
> Grazie,
> 
> 
> Eric
> 
> -- 
> 
> ===================================================
> 
>         Eric G. Wruck
>         Econalytics
>         2535 Sherwood Road
>         Columbus, OH  43209
> 
>         ph:      614.231.5034
>         cell:    614.330.8846
>         eFax:    614.573.6639
>         eMail:   ewruck@econalytics.com
>         website: http://www.econalytics.com
> 
> ====================================================
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