Limiting the number of Range of values used by a parameter
I have a parameter that allows the user to select multiple employee numbers. As an example it shows all employees from employee #1 to employee # 100. However all direct employees are are numbered from 1 to 50, contractors are 51-100. Right now they can pick any employee through 100. I want them to only be able to run the report on contractors. How do I set it up so that they can only select employee #s > 50? More direct - how do I filter a parameter list?