How many in group are named the same

[Netane Ben-Porath]

Hello,
I have data of groups and individuals. Each individuals is assigned a name. For each individual I want to know how many more individuals are named like him/her within the group.

So my data looks like the first three colums (Id, Group, Name) and I want to add the 4th column (SHARE_NAMES):

Id	Group	Name	SHARE_NAMES
1	1	JACK	2
2	1	JACK	2
3	1	ANNA	0
4	1	JACK	2
5	1	MICHAEL	0
6	2	JACK	0
7	2	NICOLE	1
8	2	MICHAEL	0
9	2	NICOLE	1

Thank you very much for any help.

[Nick Cox]

Under the aegis of by: the number of observations in any defined group is _N. So the number of others is one fewer.

* Example generated by -dataex-. For more info, type help dataex
clear
input byte(id group) str7 name byte share_names
1 1 "JACK"    2
2 1 "JACK"    2
3 1 "ANNA"    0
4 1 "JACK"    2
5 1 "MICHAEL" 0
6 2 "JACK"    0
7 2 "NICOLE"  1
8 2 "MICHAEL" 0
9 2 "NICOLE"  1
end

bysort group name : gen wanted = _N - 1

list, sepby(group name)

     +------------------------------------------+
     | id   group      name   share_~s   wanted |
     |------------------------------------------|
  1. |  3       1      ANNA          0        0 |
     |------------------------------------------|
  2. |  2       1      JACK          2        2 |
  3. |  1       1      JACK          2        2 |
  4. |  4       1      JACK          2        2 |
     |------------------------------------------|
  5. |  5       1   MICHAEL          0        0 |
     |------------------------------------------|
  6. |  6       2      JACK          0        0 |
     |------------------------------------------|
  7. |  8       2   MICHAEL          0        0 |
     |------------------------------------------|
  8. |  9       2    NICOLE          1        1 |
  9. |  7       2    NICOLE          1        1 |
     +------------------------------------------+

See also https://www.stata-journal.com/articl...article=dm0075