Sum data from two id's

[Henry Strawforrd]

I have panel data and I want to join two id' into one by adding the data. There is other groups I dont want to join (so not just collapse the whole data to the t level). How to accomplish this?

Here is an MWE. Say, I want to join id = 1 and id = 2 into one group with the values of x added together.

input id t x
1 1 10
1 2 0
1 3 5
2 1 10
2 2 15
2 3 0
3 1 10
3 2 15
3 3 0
end

[Nick Cox]

replace id = 1 if id == 2

before a collapse.

[Henry Strawforrd]

and then collapse at id and t level? Ah, thank you

[Nick Cox]

Indeed. Look at it this way: collapse can't tell what is considered as the same identifier unless it's told that somehow. In your real data there may be a systematic rule that lets you do this quickly.