In a state that does not restrict drug testing, employers may set up random tests as they see fit. To avoid discrimination claims, a random test should be truly random — every employee in the test group should have an equal chance of being selected for the random test.
That said, you can test differently according to job function. The frequency with which you do random tests and the number of employees chosen for each random test will depend on your business needs.
While there is no requirement to have a written procedure for random drug testing, documenting procedures can ensure that they are followed consistently.