Identify observations in >1 variable

[Gillian Maher]

Hello,

I wonder if anyone could help with the following:

I have 2 variables with unique identification (ID) numbers with over 30 years of follow-up time. One is ID numbers for children and the second is the ID numbers of their mothers.
Example:
ChildID:-------------MotherID:
10---------------------------20
11---------------------------21
12---------------------------10
13---------------------------22
I want to identify children who are now also mothers in the dataset. For example Child ID10 above is now also a mother. Is there a way I can create a variable to identify which children subsequently became mothers based on their ID numbers?

Thank you,

Gillian

[Andrew Musau]

There are a number of ways of doing this, but I personally like this simple and elegant method from some posts by Romalpa Akzo

input float(ChildID MotherID)
10 20 
11 21
12 10
13 22
end

expand 2, gen(x)
replace ChildID=MotherID if x
bys ChildID: egen tag= max(ChildID[_n]==ChildID[_n+1])
keep if x==0
drop x
l

Result:

. l

     +--------------------------+
     | ChildID   MotherID   tag |
     |--------------------------|
  1. |      10         20     1 |
  2. |      11         21     0 |
  3. |      12         10     0 |
  4. |      13         22     0 |
     +--------------------------+