How can I rank observations within groups in Stata?

Question

I have some data in Stata which look like the first two columns of:

group_id   var_to_rank  desired_rank
____________________________________

1           10          1
1           20          2
1           30          3
1           40          4
2           10          1
2           20          2
2           20          2
2           30          3

I'd like to create a rank of each observation within group (group_id) according to one variable (var_to_rank). Usually, for this purpose I used:

gen id = _n

However some of my observations (group_id = 2 in my small example) have the same values of ranking variable and this approach doesn't work.

I have also tried using:

egen rank

command with different options, but cannot make my rank variables make to look like desired_rank.

Could you point me to a solution to this problem?

Answer 1

The following works for me:

bysort group_id: egen desired_rank=rank(var_to_rank)

Answer 2

I'd say this question is posed the wrong way round for best understanding. The aim is to group observations, those with the lowest value all being assigned a grade 1, the next lowest being all assigned 2 and so forth. This isn't ranking in most senses that I have seen discussed, but Stata's egen, rank() does get you part of the way.

But the direct way, which was mentioned in the Statalist thread cited elewhere in this thread (start here) is simpler in spirit than any solution quoted:

bysort group_id (var_to_rank): gen desired_rank = sum(var_to_rank != var_to_rank[_n-1]) 

Once data are sorted on var_to_rank then when values differ from previous values at the start of each block of distinct values a value of 1 is the result of var_to_rank != var_to_rank[_n-1]; otherwise 0 is the result. Summing those 1s and 0s cumulatively gives the desired variable. The prefix command bysort does the sorting required and ensures that this is all done separately within the groups defined by group_id. No need for egen at all (a command that many people who only use Stata occasionally often find bizarre).

Declaration of interest: The Statalist thread cited shows that when asked a similar question I too did not think of this solution in one.