by maligned » Sat Dec 19, 2009 9:14 am
AeroDave,
The fielding bible +/- system divides the entire field into dozens of vectors and has a % system that details what percentage of balls hit to those areas at which speeds (popup, hard liners, soft liners, tweeners, etc.) are fielded.
For example, if a particular type of ball to a particular vector is converted 75% of the time by shortstops, and Jeter fields this type of ball because he was positioned well and makes an easy play; he gets +.25 credit for fielding it, even though he didn't need great range to get to it. If Tulowitzki was mis-positioned, but still dives and makes the play with phenomenal range, he gets the same credit. Similarly, if a slower range shortstop was mis-positioned and seemingly has no chance whatsoever for that same ball (watches it roll into leftfield from 3 yards away), he is penalized -.75 for not getting a ball that is fielded 75% of the time. In the end, this type of matrix is able to incorporate positioning, range, throwing arm, hands, everything.
Vector values are created using literally thousands and thousands of plays on thousands of hours of video. Then, each play for each fielder is charted and rewarded or penalized for an entire season and final +/- totals are calculated.
This has been the general philosophy for a while. The system has now been upgraded so that each +/- value considers runs saved or lost for each type of play to each vector. Each minute play or mis-play (only getting one out instead of two on a double play ball for example) is converted into a + value (runs saved) or minus value (runs lost) based on average results over hundreds of thousands of game situations. In other words, a diving play down the line by a third baseman is greater than a diving play in the hole to his left (because he saved a double instead of a single). If a double down the line is worth .5 runs to a team on average and the particular ball fielded is only fielded 10% of the time; it is worth +.45 runs if fielded (.90 over normal * .5 runs) and -.05 runs if not fielded (.10 below normal *.5 runs allowed).
Finally, each player is given a runs saved or runs lost total at the end of the season that details how many runs his every movement saved his team or cost his team. Pop-ups, grounders, bunts, double play conversion ability--everything is considered down to the greatest detail possible.
I hope this helps. Others probably know more than I do and can chime in with corrections or clarifications.