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: [email protected]
> website: http://www.econalytics.com
>
> ====================================================
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>
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