"Riano, Alejandro" wrote:
> I'm collapsing the standard deviation for a set of variables (monthly
> frequency):
>
> collapse (sd) er rer, by(country_name year)
>
> but if for example one country doesn't have data for all the year, i'm
> getting a 0 instead of a missing value after doing the collapse, do you know
> how can i solve this ?
REPLY:
Are you certain that there isn't at least one observation in the missing-data
country for the year at issue? Here is a counter-example using the auto.dta
dataset, showing that, when the variable of interest (mpg) is missing in a cell,
collapse returns sd = ., not zero.
use c:\stata\auto,replace
(1978 Automobile Data)
. drop if rep78 <3 | rep78==. [to simplify the dataset]
(15 observations deleted)
. replace mpg = . if foreign ==0 & rep78 ==5 [to setup a cell with missing
mgp]
(2 real changes made, 2 to missing)
. tab foreign rep78
| Repair Record 1978
Car type | 3 4 5 | Total
-----------+---------------------------------+----------
Domestic | 27 9 2 | 38
Foreign | 3 9 9 | 21
-----------+---------------------------------+----------
Total | 30 18 11 | 59
. tab foreign rep78, sum(mpg)
Means, Standard Deviations and Frequencies of Mileage (mpg)
| Repair Record 1978
Car type | 3 4 5 | Total
-----------+---------------------------------+----------
Domestic | 19 18.444444 . | 18.861111
| 4.0856221 4.5856055 . | 4.1552224
| 27 9 0 | 36
-----------+---------------------------------+----------
Foreign | 23.333333 24.888889 26.333333 | 25.285714
| 2.5166115 2.7131368 9.367497 | 6.3098562
| 3 9 9 | 21
-----------+---------------------------------+----------
Total | 19.433333 21.666667 26.333333 | 21.22807
| 4.1413252 4.9348699 9.367497 | 5.8979946
| 30 18 9 | 57
. collapse (sd) sdmpg = mpg , by( foreign rep78)
. list
rep78 foreign sdmpg
1. 3 Domestic 4.08562
2. 4 Domestic 4.58561
3. 5 Domestic .
4. 3 Foreign 2.51661
5. 4 Foreign 2.71314
6. 5 Foreign 9.3675
john
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