[n4r9]

Surely it's meaningless to compare the efficiency of slavery vs other systems, since your set of resources is completely different.

[tgv]

You could if you look at e.g. the crop yield (ceteris paribus). I don't know why you would, because what sane conclusion could you draw from it?

[direwolf20]

You could compare systems to identify which one produces the greatest profit from the least costs, which the main thing an entrepreneur cares about.