Monday, August 21, 2006

How good are the best fielders?

How many runs is a great fielder worth? There are lots of rating systems and lots of different answers. Tangotiger (Tom) takes a stab at the answer, but instead of analyzing play-by-play data, he just goes with his gut.

He starts by dividing batted balls into degree of difficulty. About 70% are “automatic,” and there should be little if any difference between good and bad fielders -- Tom figures the good fielders convert 99% of those plays, and the bad fielders convert 97%. On plays that take “some effort,” Tom estimates 80% vs. 50%. On difficult plays, it’s 50% vs. 15%. And on “highlight reel” plays, he estimates that great fielders convert five times as many of those plays as bad fielders do: 15% to 3%.

Do the arithmetic, and the difference between a good and bad fielder is 10%, or 60 plays a season. That is, the best fielders are 5% above average, and the worst are 5% below average. It’s higher for a shortstop, because that position sees 30% more balls in play -- so figure 40 plays above average instead of 30.

What it boils down to, basically, is that if 70% of plays are easily handled even by bad fielders, there’s not much room left in the other 30% to make more than 30-40 plays worth of difference.

Tom welcomes you to substitute your own numbers -- but they have to meet the constraint that, on average, 70% of plays get turned into outs, since that’s the MLB average conversion rate. He says that the highest he can go on reasonable assumptions is maximum 42 plays above average, which is 55 for a shortstop.

(What he doesn’t mention is that some shortstops might see even more plays, due to an overbalance of left-handed pitchers, or groundball pitchers, or just plain luck. But the 5% figure would still hold, even if the number of plays winds up a bit higher.)

Now, obviously, this kind of analysis is not an evidence-based, empirical study. But it helps a lot. It gives us a general range that we expect any result to fall into. If someone comes out with a study that purportedly shows that a second baseman can save his team 100 hits per year, we now have a basis to question it. I agree with Tom that it’s very difficult to save that many hits -- with 70% of balls in play being automatic, that leaves only 225 plays remaining in which to make a difference. It’s very difficult to watch a game and pick up one-and-a-half balls a game that (a) are hit in the vicinity of the second baseman, (b) are difficult plays, (c) that the fielder still manages to turn into an out, and (d) an average second baseman would almost certainly not have made. Our general observations, while not in the same league as good empirical evidence, are still pretty good, good enough to justify a strong skepticism.

And it seems to me that 40 plays a year is about right. Turning 40 hits into outs is equivalent to about 80 points in batting average for a regular player. If the average SS hits .270, even the best fielding shortstops won’t be in the lineup if they hit .190. And Derek Jeter, whose defense has a horrible reputation in sabermetric circles, just happens to be more than 40 singles above average offensively.




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