Generating a True-False Binary from by virtue of the order of events

[Matthew Record]

Hello all,

I have a dataset that has an ID indicator, a year and an X variable. The data looks like so:

ID	Year	X1
1	2005	A
1	2011	B
2	2006	B
2	2007	A
3	2003	A
3	2012	B
4	2001	B
4	2003	A
5	2005	B
5	2009	A

For some units, "A" occurs first chronologically and for others, "B" occurs first chronologically. I want to generate a binary variable by ID that will be 1 if A occurs first chronologically and 0 if B occurs first. I'm sure this is a bysort: egen operation but I'm having a hard time wrapping my head around how to accomplish this.value

Thank you very much in advance!

[Maarten Buis]

. clear

. input id        year    str1 x

            id       year          x
  1. 1       2005    A
  2. 1       2011    B
  3. 2       2006    B
  4. 2       2007    A
  5. 3       2003    A
  6. 3       2012    B
  7. 4       2001    B
  8. 4       2003    A
  9. 5       2005    B
 10. 5       2009    A
 11. end

. bys id (year) : gen byte afirst = x[1] == "A"

. list, sepby(id)

     +------------------------+
     | id   year   x   afirst |
     |------------------------|
  1. |  1   2005   A        1 |
  2. |  1   2011   B        1 |
     |------------------------|
  3. |  2   2006   B        0 |
  4. |  2   2007   A        0 |
     |------------------------|
  5. |  3   2003   A        1 |
  6. |  3   2012   B        1 |
     |------------------------|
  7. |  4   2001   B        0 |
  8. |  4   2003   A        0 |
     |------------------------|
  9. |  5   2005   B        0 |
 10. |  5   2009   A        0 |
     +------------------------+

[Matthew Record]

Worked great! Thank you so much!