Random sampling of records

A very low tech solution would be to use the Record ID if the Employee record and test for even or odd. That will give you about 50%. 

Extending that thought, you can make a formula field 

Rem([Record ID#], 3)

which will return a 0, 1, or 2  that would let you select 1/3rd or 2/3rds  

Obviously, you can use sny number in the REM function, like 10 to sample different percentages