Don't forget
treatment bias, also known as "nature versus nurture". One can totally remove sampling and selection bias by tallying the
whole population in the measurement, say in antiquity. I predict the data (which can't have sampling and selection bias) would describe how slaves have a higher injury and mortality rate: after whipping or overworking or being fed inferior food, the injured are last in line when the food is dispensed etc. This is real world
treatment bias. How
just is it to increase say insurance rate for the slaves? (I know they probably didn't have insurance in antiquity...)
Edit:
In other words, statistics done correctly (i.e. representatively) on the real world can tell us what the real world (and its status quo) looks like, but tells us nothing about the ethics of the situation.