Range factor measures how many plays he makes. It's only use is to compare it to other SS's on his team, as the pitching staff dictates how many ground balls he'll see. Zone rating measures how many balls were in "his zone" and how often they were turned into outs. You still have the human element of the stats people, but overall, ZR is far superior. If I played SS with 4 LHP in the rotation, and 3 of them were sinker specialists, i might have a higher RF than JJ, and though I have a decent glove, you can measure my speed with a sun dial. However, my ZR would be terrible.