How to complete (fill) a panel dataset with a time variable that has a delta > 1?

Say I have a bi-yearly panel only with observations at odd years, such as

input id year var
       1 2011  23
       1 2013  12
       1 2015  11
       
       2 2011  44
       2 2013  42
       2 2015  13
end

and I would like to fill up the missing even years. Here years 2012 and 2014 is missing for all ids.

input id year var
       1 2011  23
       1 2012   .
       1 2013  12
       1 2014   .
       1 2015  11
       
       2 2011  44
       2 2012   .
       2 2013  42
       2 2014   .
       2 2015  13
end

I had a look at help expand but I am unsure that's what I need, since it does not take the by prefix.
As a background info, I need to fill up with even years to able to merge with another panel data-set conducted in even years only

Solution

You can set the panel id as id and the time variable as year and use tsfill:

clear
input id year var
       1 2011  23
       1 2013  12
       1 2015  11
       
       2 2011  44
       2 2013  42
       2 2015  13
end 


xtset id year
tsfill

If the min and max year is not constant across panels, you could look at the ,full option.


. list

     +-----------------+
     | id   year   var |
     |-----------------|
  1. |  1   2011    23 |
  2. |  1   2012     . |
  3. |  1   2013    12 |
  4. |  1   2014     . |
  5. |  1   2015    11 |
     |-----------------|
  6. |  2   2011    44 |
  7. |  2   2012     . |
  8. |  2   2013    42 |
  9. |  2   2014     . |
 10. |  2   2015    13 |
     +-----------------+