st: Creating square matrix with obs counts from 2 non unique id variables

[Diogo L Pinheiro <diogo.pinheiro@pubpolicy.gatech.edu>]

Hello, everyone,
I've been trying to create a square matrix with counts of observations 
from two non unique ID variables. In this case, one id variable has the 
code for an organization an individual was and one for where the 
individual is. For example:


Observation    ID1         ID2
1.                  10          12
2.                  10          11
3.                  11          12
4.                  11          10

In the example above, observation 1 was at organization #10 at one time 
and organization #12 at another. Observation 2 was at 10 and then 11. 
And so on.


From that, I want to create a square matrix that would look something like:


              10           11           12

10           0              1            1
11           1              0            1
12           0              0            0

So that I could have all the counts of each combination of organizations.

The main problem is getting a square matrix in the end. I can create a 
non square matrix in this case by using
tab ID2, gen(newID)
collapse (sum) newID, by(ID1)

but the problem is that this won't be a square matrix, as not every ID 
number shows up in both variables.. In the example above it would be 
missing a line (since not every ID number present in ID1 is present in 
ID2, and vice versa).

So how could I get a square matrix from the situation above? I've found 
commands to create similar matrices, but they require unique IDs and 
only provide binary results for the existing combinations.


Thanks!
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