Hi

How to drop whole "person_id" if for the same treatment there is NOT time = 1 & time = 3 & time = 6 & time = 12 & time = 18? Please see example below:

Code:
person_id       treatment           time
      1                 A             1
      1                 A             3
      1                 A             6
      1                 A             12
      1                 A             18
      2                 B             1
      2                 C             3
      2                 C             6
      2                 C             18
      3                 A             6
In this case, I would like to have (dropping all the other observations):

Code:
person_id        treatment            time
      1                 A             1
      1                 A             3
      1                 A             6
      1                 A             12
      1                 A             18
Thanks!!