keep variable in certain condition

[hatsune miku]

hello everyone, I want to keep variable maternal labor supply only in 2009 as covariance at baseline. could you please give me any code or advice?

thank you!

[Carlo Lazzaro]

Hatsune:
please explain what you're after. Thanks.

[hatsune miku]

Hi Carlo!
I want to find the effect of child enrollment in preschool on the EDI score, and maternal labor supply as a control variable, in the data set, the EDI score only existed in 2013 (endline) which I will use as the dependent variable, but for the maternal labor supply variable it is still there is data for 2009 (baseline), 2010 (midline), and 2013 (endline) whereas I only want to use maternal labor supply data available in 2009 (baseline).

thank you for your answer.

[Carlo Lazzaro]

Hatsune:
an -if- clause can do the trick, as in the following toy-example:

 use "C:\Program Files\Stata17\ado\base\a\auto.dta"
(1978 automobile data)

. regress price mpg i.rep78 if rep78==3
note: 3.rep78 omitted because of collinearity.

      Source |       SS           df       MS      Number of obs   =        30
-------------+----------------------------------   F(1, 28)        =     14.65
       Model |   123788982         1   123788982   Prob > F        =    0.0007
    Residual |   236582733        28  8449383.33   R-squared       =    0.3435
-------------+----------------------------------   Adj R-squared   =    0.3201
       Total |   360371715        29  12426610.9   Root MSE        =    2906.8

------------------------------------------------------------------------------
       price | Coefficient  Std. err.      t    P>|t|     [95% conf. interval]
-------------+----------------------------------------------------------------
         mpg |  -498.8875   130.3389    -3.83   0.001    -765.8747   -231.9003
     3.rep78 |          0  (omitted)
       _cons |   16124.28    2587.92     6.23   0.000     10823.17    21425.39
------------------------------------------------------------------------------

.

[hatsune miku]

how to use it for multiple variables? I mean maternal labor supply is just an example of my covariance variable.

[Carlo Lazzaro]

Hatsune:
1) you can use multiple -if- conditions; or
2) create new variables that include the year your intrested in:

gen mother_work_2009=mother_work if year==2009

[hatsune miku]

I did your suggestion to generate a new variable, after I did the sum there was a difference in the number of observations, and I tried to drop the missing value but instead deleted all observations.

[Carlo Lazzaro]

Hatsune:
Stata uses listwise deletion by default.
Hence:

 
 gen mother_work_2009=mother_work if year==2009

will generate missing values when year differs from 2009.

No wonder that you remain with zero observations, then, as listwise deletion gets rid of all the observations with missing value in any of the variables.