Simple systems are simple to enforce. The more granular the system the more overhead to sort out and the more mistakes / corruption.

1) Listing the different numbers of jobs and what categories they are in.
2) The length of time people are doing each section of jobs.
3) Different levels of stress in non-physical jobs as this too affects stress levels.
4) Different people have different tolerances to different things. Better factor that in too to make sure the system is as fair as possible.