Count number of different values with group-command

[Fabia Junker]

Hello members,

I would like to find out how many different values there are for the variable betnr (establishment number) per year.
To do this, I used the command "egen count_betnr = group(betnr year)" to count the number of different establishment numbers per year.
However, I then get different numbers for the same year. How can this be explained?

Is there another approach to count the number of different establishment numbers per year?

[Nick Cox]

See e.g.

SJ-23-2 dm0042_4  . . . . . . . . . . . . . . . . Software update for distinct
        (help distinct, distinctgen if installed)  N. J. Cox and G. M. Longton
        Q2/23   SJ 23(2):595--596
        most important change is addition of distinctgen command

SJ-20-4 dm0042_3  . . . . . . . . . . . . . . . . Software update for distinct
        (help distinct if installed)  . . . . . .  N. J. Cox and G. M. Longton
        Q4/20   SJ 20(4):1028--1030
        sort() option has been added

SJ-15-3 dm0042_2  . . . . . . . . . . . . . . . . Software update for distinct
        (help distinct if installed)  . . . . . .  N. J. Cox and G. M. Longton
        Q3/15   SJ 15(3):899
        improved table format and display of large numbers of
        observations

SJ-12-2 dm0042_1  . . . . . . . . . . . . . . . . Software update for distinct
        (help distinct if installed)  . . . . . .  N. J. Cox and G. M. Longton
        Q2/12   SJ 12(2):352
        options added to restrict output to variables with a minimum
        or maximum of distinct values

SJ-8-4  dm0042  . . . . . . . . . . . .  Speaking Stata: Distinct observations
        (help distinct if installed)  . . . . . .  N. J. Cox and G. M. Longton
        Q4/08   SJ 8(4):557--568
        shows how to answer questions about distinct observations
        from first principles; provides a convenience command

where the 2008 paper remains the fullest discussion (that I know of) and contains a simple two-step using official code only:

egen tag = tag(betnr year)
egen distinct = total(tag), by(year)

followed by e.g.

tabdisp year, c(distinct)

This is equivalent:

bysort year (betnr) : gen wanted = sum(betr != betr[_n-1]) 
by year : replace wanted = wanted[_N]

[Fabia Junker]

Thank you Nick!