_n not working as a running counter within a by-group

[Francisco Pardo]

Dear Statalist,

I normally use _n within a by-group as the running counter within the group. Examples of this are option 1 and 2. However, it appears not to be working in option 3 as it should. It seems to me it should give same results as option 1 but it appears that the 'max' function is taking the running counter of the complete database. I include option 2 as to show this is not the case for other functions in egen.

Any ideas?

clear
set obs 10
gen group=ceil(_n*2/_N)

gen n=_n
sort group n

*Option 1
by group: gen n1=_n //Works as running counter within a by-group
by group: egen o1=max(n1)

*Option 2:
by group: egen o2=mean(_n) //Works as running counter within a by-group

*Option 3:
by group: egen o3=max(_n) //Works as running counter within the complete dataset

     +--------------------------------+
     | group    n   n1   o1   o2   o3 |
     |--------------------------------|
  1. |     1    1    1    5    3    5 |
  2. |     1    2    2    5    3    5 |
  3. |     1    3    3    5    3    5 |
  4. |     1    4    4    5    3    5 |
  5. |     1    5    5    5    3    5 |
  6. |     2    6    1    5    3   10 |
  7. |     2    7    2    5    3   10 |
  8. |     2    8    3    5    3   10 |
  9. |     2    9    4    5    3   10 |
 10. |     2   10    5    5    3   10 |
     +--------------------------------+
     

Thanks!

Francisco