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The long season: Changes in fielding from April to October

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Each baseball season lasts a long time and it stands to reason that players perform differently at the beginning and end of the season. A look at whether fielding ability changes and how.

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The baseball season is long and at no point in time is that more evident than August. The excitement of the trade deadline has passed, and meager tidbits like "will the Rockies waiver-trade Jose Reyes now, or wait until the end of the year?" are what pass for sustenance on that front. The playoffs are still several months away, but for the vast majority of teams, their fate is sealed, or close enough to sealed that it robs games of any drama. For the fans of the six to eight teams on the brink, August must feel like it's taking an eternity, magic numbers falling glacially, suspense compressing oh so slowly into certainty.

Imagine what it feels like to be a player at this time. Obviously, playing is a much different experience, but the boredom and/or tension are almost certainly present as well, but instead of a pastime they can pay more or less attention to as they please, it's their job; a job that involves physical labor ten of every eleven days, with a single three day vacation -- unless you're performing well, in which case, congratulations, you get to play more!

It sounds brutal, "it" being what Russell Carleton termed "the Grind." In his article at Baseball Prospectus, Carleton found that as the season went on, players' plate discipline eroded, swinging or taken more strikes and fewer balls, presumably the impact of the aforementioned brutality of a season that lasts half a year. Plate discipline is not a totally surprising thing to be impacted, in that the margins between success and failure are minuscule. That description applies somewhat to every aspect of baseball, but in thinking about other areas in which fatigue, boredom, or ennui could have a particular impact, I came to fielding.

Fielding is another area like plate discipline where small lapses in judgement or a slight slowing of the reflexes can have big impacts.

Before I begin, however, a massive disclaimer: the other reason plate discipline is the perfect aspect to examine over the course of a season is the sheer number of trials. With every single pitch in a season counting toward or against the existence of the hypothesized impact, evidence (presumably) wasn't too hard to find. There are significantly fewer defensive opportunities, however, so I would take any conclusions that follow with a grain of salt. Unlike Russell's article above, however, I'm not trying to pull out individual contributions, I've analyzed the aggregated data, which hopefully avoids too much sample-size interference.

I'm using the DEF rating from FanGraphs, which combines a player's estimated fielding ability with the positional adjustment, and converting it to a rate per 600 PAs. The data is combined, using each year from 2010 through 2014. I proceeded month by month, and treated it like an aging curve, calculating the change in DEF from month to month and weighting each year by the harmonic mean of the PAs in each month.

Hm. Rather than the hypothesized decline, that shows a fairly steady increase over the course of the season. Perhaps rather than becoming exhausted, players take several months to fully loosen up. To be fair, it appears to be less than a run per season of difference between the beginning of the season and the peak, so it's barely a noticeable shift. But over several seasons, it's there.

To try to determine exactly what's motivating the change, I decided to drill down a little deeper, and look at the same data but for individual positions. The positions further along the defensive spectrum generally seem more physically challenging, and therefore more likely to exhibit the effects of wear over the course of the season, if that were actually a thing. For reasons that will soon become clear, I'm going to present them in order of position: C, 1B, 2B, 3B, SS, LF, CF, RF.

The changes over the season are decidedly not uniform. Almost no position experiences a steady decline, not even catcher, but each of the outfield positions sees a steady increase in defensive performance, finishing more than a run per 600 PAs above where they started.

I have no explanation for this! If anything, I would think outfield is harder than the infield, with more running and walls to bang into, and more likely to grind a player down over the course of the season. If that is the case, however, it's not enough to impact the defensive ability of outfielders, and there's something else that's making them improve as the season goes on, something that doesn't affect infielders. Maybe outfielders take longer to get loose, maybe it's an atmospheric thing, and with more outfield plays coming on balls in the air and more infield plays on grounders, the outfielders benefit while the infielders don't.

Speculating like this isn't too helpful, but this insight isn't in-depth enough to allow for anything more. Certainly part of the lack of any well-defined conclusion is the lack of detailed, transparent defensive metrics. That said, the data does seem to show that there is a real effect on fielding over the course of a season, on outfielders if not infielders. Statcast might not be ready yet, but it's coming, and hopefully it will move questions like this into the "answerable" category sometime in the near future.

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Henry Druschel is a contributor at Beyond the Box Score. You can follow him on Twitter at @henrydruschel.