Over the last few weeks, I’ve converted the fielding stats available at The Hardball Times into a little plus/minus stat, similar to what others have done. Although Justin gives you a good, concise explanation on the methodology (and here’s his more detailed post on the subject, which I’d highly recommend), I wanted to try it myself and take you through an example. If you’re familiar with the process, which many of you may be, there’s probably nothing new in this post …
THT buys the data from Baseball Info Solutions, and they give you a few things to work with:
Balls in zone (BIZ) — This is the number of balls hit into a player’s zone over the course of the season. A player’s zone(s) is the area where fielders at that position convert at least 50% of balls into outs (more from Dave Studeman).
Plays — Number of balls converted into outs on balls in zone.
Revised zone rating (RZR) — This is simply: plays/BIZ. So if Khalil Greene has 300 balls hit into the shortstop zone and converts 250 of them into outs, his RZR is .833.
Out of zone plays (OOZ) — This is the number of outs a player makes on balls hit outside of his zone.
So, let’s runs through a little example of how to turn these four numbers into a plus/minus stat (all numbers are made up):
Find the average RZR at each position (let’s use shortstop here): .850
Based on the avg. RZR at that position, find the number of plays a player should have, based on his BIZ: Khalil — 300 BIZ * .850= 255 plays made. Let’s say he actually has 265 plays made, so he’s plus 10 plays on balls in zone.
Out of zone:
Find average number of out of zone plays per BIZ at short: .135
Based on that number (.135), calculate the number of out of zone plays a fielder should have, based on his BIZ: Khalil — 300 BIZ * .135 = 40.5. Let’s say he actually has 40 plays, so he’s -1 play on out of zone balls.
Now add the two numbers together to get a total number of plays above/below average: Khalil: 10 – 1 = 9 plays above average. Since you generally want things measured in runs, rather than plays, convert to runs using Chris Dial’s plays to runs conversions. You may notice those numbers make a lot of sense, intuitively (at least to me): a play saved at short is worth ~.75 runs, while a play saved in center is worth ~.842 runs. Why? An out is an out, right? Well, hits that get by the shortstop are almost always singles and base runners usually gain only one or two bases. On the other hand, many hits into center are doubles or triples and base runners can more often take two or three bases.
Anyway, to finish it out, multiply plays above average by Dial’s number: Khalil: +9 plays*.753= +6.8 runs above average
That’s how I did it. Not too complicated, I don’t think — but please let me know if you have any questions or if I made any mistakes. What follows is some of the positives and negatives of this stat (in my opinion).
Why I like this ’stat’
- It’s based on play by play data. Some fielding stats, like Range Factor or Baseball Prospectus’, aren’t based on play by play data. Baseball Prospectus uses a bunch of adjustments to try and estimate how many chances a player had. With THT/BIS’ data, you don’t have to do that, because it is based on play by play data. So you’ve got a pretty darn good idea of how many balls were in a player’s zone, or were “fieldable.”
- It’s based on performance both in and out of zone. I see a lot of people using RZR to measure fielding and while that’s great, I think you’re missing too much of the total picture (the OOZ balls) when you do that. So with some relatively easy calculations, you can turn it into a plus/minus stat, using both RZR and OOZ.
- There are not many (any, really) adjustments, like you’ll see in something like UZR. Those adjustments may be for park, pitcher handedness, batted ball speed, and so on. Obviously, this stat isn’t on the level of a more detailed fielding metric like UZR or John Dewan’s plus/minus, but it’s free and I think it’s pretty darn good : )
- A ball is either determined to be “in zone” or “out of zone.” Take, for instance, a ball hit right at the shortstop. That ball may be converted into an out, say, 90% of the time. Now, take one hit on the fringe of the SS zone; maybe it’s converted into an out 55% of the time. Those balls appear to be equal, under this system (since they were both hit in the SS zone). Over the course of a year, you expect stuff like that to even out, and fielders to have a relatively balanced distribution of opportunities, but surely it doesn’t always (or, even, often) happen.
- There are no “opportunities” for out of zone plays. We know how many OOZ plays a fielder makes, but we don’t know exactly how many chances he had. I (like Justin and others) use in zone chances as a proxy to estimate OOZ chances, but this certainly may not be the best way to do it — maybe it is balls in play (or ground balls for infielders; fly balls for out fielders, etc.), maybe innings, I don’t know. Either way, it is an issue but I’m not sure of its significance.
- It is not an all-inclusive fielding stat. For outfielders it doesn’t include throwing arms, for infielders it doesn’t include double plays or line drives/pop ups, for first basemen it doesn’t include catching throws from other fielders, etc.