It s really a strong network of data collection, but it is not perfect. Every player has one, and it is close to the fraction of balls a player converts into outs divided by balls hit into a player s zone. A player gets a bonus for getting to balls outside his zone. For infielders, imagine a 20-foot circle around each player. That s the area each player is expected to cover.
All shortstops are responsible for the same zones.
Since they all are assigned the same zones, and they all stand within a few feet of each other on the field, the percentage of balls they turn into outs should be a mostly accurate description of how well they field. It will undervalue defense because it doesn t consider the ability to get to pop-ups and turn double plays. If you need to make a judgement on how good a fielder is, try to use ZR at a minimum. Unfortunately, there is no ZR data older than about 1985. That means the Gold Gloves, the fielding percentages and range factors for all the older players will have to suffice.