Get the doctor to assess the nurse. Or the head nurses if you don't trust doctors. The nurses have managers, and if none of the doctors or head nurses can be trusted with a simple matter like assessing whether nurses are doing their jobs then you got bigger issues.
Oh no, the boss might play favourites if it's not an objective measure! Oh the injustice /s
But stupid rules or KPI also allow favourites. You can use an officious 30 point checklist and play favourites while ticking boxes. You can even rig "objective" data by controlling other factors (e.g. giving someone difficult customers do deal with).
Yeah, data driven would be nice, if you have good data. But data driven is a power tool. You don't measure SLOC or reward token use in software because of perverse incentives.