A statistically minded baseball fan typically does not have to go far to find criticisms of the defensive metrics available. Certainly, some of that criticism is valid – defensive metrics are by no means perfect, and they particularly have flaws when looking at single seasons of data. This piece at Baseball Prospectus (subscription required) by Colin Wyers does a fantastic job of framing the problems and biases involved with defensive metrics, as well as a possible answer.
One issue presented with defensive metrics I believe is invalid, however, is the idea that inconsistencies in defensive ratings from year to year somehow render defensive systems useless, whether it be UZR, TotalZone, +/-, or any other stat. The idea seems to be based on a traditional baseball idea that while pitching and hitting can slump, speed and defense both tend to hold constant. According to this idea, seeing a player post UZRs of -3, +6, -9, and then +2 in four consecutive seasons would represent a problem with the metric as opposed to simply the ups and downs of single season. Such fluctuations in wRAA aren’t uncommon, but such things can be explained away by hot streaks and slumps over the course of multiple seasons.
Sky Andrecheck analyzed the idea of “defense doesn’t slump” last year. Based on the idea that the distribution of probabilities of outs on balls in play is bimodal – that is, most are either sure hits or sure outs – the standard error for fielders is smaller than that for hitters.
From an individual player’s standpoint, the average fielder has about 500 balls in play in his area over the course of the season (of course, this varies by position, and we can adjust accordingly) . Using the numbers above, we see that the average fielder has a standard error of about .23*SQRT(500) = 5.14 outs over the course of a season. This means that he is prone to make about 5 or so more or 5 or so less plays in a season than his true talent would usually call for. This corresponds to a difference of about 4 runs in a season. While this is fairly small, it does show that random variability can play a part in a fielder’s performance just as it can for hitters
Sky’s work, to me, effectively proves that we should expect some variability in defensive abilities. There’s also another element to the variation we see in defensive metrics that he doesn’t look at, and that’s the physical and mental aspects of the game. If a player tweaked his hamstring but doesn’t tell his manager, we could see a decrease in his range that would be unexplained by the information available. If a player is uncomfortable in a certain park, for whatever reason, he could slump if he receives an abnormal amount of chances at that park in that season. There are probably a multitude of other reasons as well.
The issue of chances is likely one of the reasons that we don’t typically recognize fielding slumps. I posed the question of why slumps can’t occur on defense on twitter, and the omnipresent Colin Wyers pointed out that it is “because a player’s ability to get to a batted ball informs our thinking about whether or not he should have made a play.” A player who is slumping on defense won’t get to balls that he may have gotten to normally, and that’s terribly difficult for fans to point out. Announcing crews aren’t going to have convenient stats like “Prince Fielder has one hit in his last 24 at bats” for defense.
Basically, I think that there’s plenty of reason to believe that defense slumps, and although we don’t really know the magnitude, I also don’t see why it wouldn’t be near that of the slumps we see for batters, or even greater due to the fact that all defensive chances aren’t created equal nor distributed at a regular schedule. Similarly, I would imagine that defense sees hot streaks as well – perhaps my focus on poor fielding is because I’m a pessimist. Regardless, as our ability to evaluate defense evolves, I predict that we won’t look at fluctuations in defensive metrics as a sign of incorrectness, and instead we will learn to accept that, for whatever reason, it’s not fair nor reasonable to accept that a player is the same quality defender against every ball in play in every game in every season.