Find sum for each year,id in panel data

[Jason Cruso]

Hello Stata Xperts!

Need some help.
I have the following panel data.
Year = year
Id = Identifier of the company
Emp = Total number of employees in the firm

I need to find the total number of employees for each year. i.e. in 1990, there are total 100 employees, while in 1991, there are total 250 employees.

clear
input float(Year Id Emp)
1990 1 25
1990 1 25
1990 1 25
1990 2 75
1990 2 75
1991 1 50
1991 1 50
1991 2 200
1991 2 200
1991 2 200

I would like to find the sum of the number of employees (emp) for each year. i.e. for 1990, the total number of employees is 100 (25 + 75). Similarly, for 1991, the total number of employees is 250 (50 + 200). Thanks for your help, J

[Clyde Schechter]

Your data structure is a bit bizarre, because you have all these redundant duplicate observations. The simplest approach would be:

duplicates drop 
by Year, sort: egen total_employees = total(Emp)

On the assumption that there is some reason why you actually need to retain all those duplicate observations, an alternate approach would be:

egen flag = tag(Year Id)
by Year, sort: egen total_employees = total(cond(flag, Emp, .))

[Jason Cruso]

Thanks Clyde. Actually I do need those observations. Your code works. Thanks a lot!