keep one observation within a group

[River Huang]

Dear All, Suppose that I have a dataset

* Example generated by -dataex-. To install: ssc install dataex
clear
input long id str1 typrep str3 indu float(roa roe bsdt)
1 "A" "J66" .008948 .294976 17622
1 "A" "J66"       .       . 17713
1 "B" "J66"  .00971 .297848 17713
1 "A" "J66" .010469 .306175 17805
1 "B" "J66"       .       . 17805
1 "A" "J66" .001485 .041761 17897
1 "A" "J66" .001572 .047049 17987
2 "A" "K70" .069677 .203744 17622
2 "B" "K70" .042736 .090677 17622
2 "A" "K70"       .       . 17713
2 "B" "K70" .088329 .181697 17713
2 "A" "K70" .054147 .176788 17805
2 "B" "K70" .075659 .152047 17805
end
format %td bsdt

There are twp kinds of reports, `typrep'=A,B. I wish to keep only one type of report A or B for each `id' and `bsdt'.

1. If `roa' is available for both A and B, choose A.
2. If `roa' is missing for B, keep A.
3. if `roa' is missing for A, Keep A.

Any suggestions? Thanks.

[Jorrit Gosens]

bys id bsdt (typrep): keep if _n==1

[River Huang]

Dear Jorrit, My bad. I had a typo. In 3, it should be if `roa' is missing for A, Keep B.

[Jorrit Gosens]

Yeah that makes more sense actually.
here's code with an example of where situations exist where: A is missing but it is the only obs by id and date, and A and B are both missing for a id and date.
In both cases, A is preserved with a missing value for roa.
Is that what you would want?

* Example generated by -dataex-. To install: ssc install dataex
clear
input long id str1 typrep str3 indu float(roa roe bsdt)
1 "A" "J66" . .294976 17622
1 "A" "J66"       .       . 17713
1 "B" "J66"  . .297848 17713
1 "A" "J66" .010469 .306175 17805
1 "B" "J66"       .       . 17805
1 "A" "J66" .001485 .041761 17897
1 "A" "J66" .001572 .047049 17987
2 "A" "K70" .069677 .203744 17622
2 "B" "K70" .042736 .090677 17622
2 "A" "K70"       .       . 17713
2 "B" "K70" .088329 .181697 17713
2 "A" "K70" .054147 .176788 17805
2 "B" "K70" .075659 .152047 17805
end
format %td bsdt

egen roamax = max(roa), by (id bsdt)
bys id bsdt: drop if roa==. & roamax!=roa & _n==1
bys id bsdt (typrep): keep if _n==1

[River Huang]

Dear Jorrit, Thanks for the suggestion. Similar results can be obtained by the following code as well.

bysort id bsdt (typrep): egen n = count(roa)
drop if typrep == "B" & n == 2
collapse roa roe, by(id bsdt)