My question is very related to the question below:
I want to generate a variable only including the top 4 firms with largest sales and exclude the rest.
In other words the new variable will only have values of the 4 firms with largest sales in a given industry for a given year and the rest will be .
Consider this:
webuse grunfeld, clear
bysort year (invest) : gen largest4 = cond(_n < _N - 3, ., invest)
sort year invest
list year largest4 if largest4 < . in 1/40, sepby(year)
+-----------------+
| year largest4 |
|-----------------|
7. | 1935 39.68 |
8. | 1935 40.29 |
9. | 1935 209.9 |
10. | 1935 317.6 |
|-----------------|
17. | 1936 50.73 |
18. | 1936 72.76 |
19. | 1936 355.3 |
20. | 1936 391.8 |
|-----------------|
27. | 1937 74.24 |
28. | 1937 77.2 |
29. | 1937 410.6 |
30. | 1937 469.9 |
|-----------------|
37. | 1938 51.6 |
38. | 1938 53.51 |
39. | 1938 257.7 |
40. | 1938 262.3 |
+-----------------+
If you had missing values, they would sort to the end of each block and mess up the results.
So you need a trick more:
generate OK = !missing(invest)
bysort OK year (invest) : gen Largest4 = cond(_n < _N - 3, ., invest) if OK
sort year invest
list year Largest4 if Largest4 < . in 1/40, sepby(year)
With this example, which you can run, there are no missing values and the results are the same.