Counting number of persons graduating each year in long-format data

[Emil Alnor]

My data is time series individual data in long format. I have a variable 'pnr' showing the id of the person, 'year' showing the year, 'degree' showing the type of highest obtained degree of the individual, and 'gradyear' showing the year in which the individual obtained their highest degree. My data looks something like this:

* Example generated by -dataex-. For more info, type help dataex
clear
input byte(pnr year gradyear education)
1 1 . .
1 2 . .
1 3 3 1
1 4 3 1
2 1 1 2
2 2 1 2
2 3 1 2
2 4 1 2
3 1 1 1
3 2 1 1
3 3 1 1
3 4 4 2
end

I need to count the number of individuals graduating in each year. A

tab gradyear

won't do because gradyear is stated for each year in the data, so here the graduation year '3' will have a frequency of 2 even though its only person 1 who graduated in year 3. So how do I count the number of occurences of gradyear==3 (and the other year) but only count one occurence pr. person?

[Hemanshu Kumar]

duplicates drop pnr gradyear, force
tab gradyear

[Øyvind Snilsberg]

alternatively,

egen tag = tag(pnr gradyear)
tab gradyear if tag

[Nick Cox]

.

 egen tag = tag(pnr gradyear) 

. tab gradyear if tag 

   gradyear |      Freq.     Percent        Cum.
------------+-----------------------------------
          1 |          2       50.00       50.00
          3 |          1       25.00       75.00
          4 |          1       25.00      100.00
------------+-----------------------------------
      Total |          4      100.00

[Emil Alnor]

Thanks all, for your solutions!