Roster Expansion and September Hitting

Over the last few weeks, several players who have had pretty lousy years have gotten hot at the same time. For instance, here are the September lines for some various players:

Ichiro Suzuki: .424/.452/.593, 207 wRC+
Carlos Pena: .293/.473/.512, 172 wRC+
Gaby Sanchez: .311/.436/.556, 170 wRC+
Justin Smoak: .345/.419/.527, 165 wRC+
Justin Upton: .312/.365/.558, 147 wRC+
Russell Martin: .246/.358/.491, 136 wRC+

Ichiro was essentially give away by the Mariners at the trade deadline (at his request), a sign of just how far he’d fallen from his time as the franchise icon. Pena, Sanchez, and Smoak all played themselves out of jobs earlier this year. Upton struggled to the point that Arizona made him available for trade talks, and is widely expected to move him this winter. And, while Martin has shown decent power for a catcher, his average has hovered around the Mendoza Line all season.

When you mention that these kinds have had a good run the last few weeks, there are generally two responses:

A. Small Sample Size, which, well, yes, of course it is.

B. September hot streaks should be discounted because of roster expansion, as inferior non-MLB pitchers are taking the hill and skewing offensive numbers around the sport.

There’s no arguing with the first point, as any monthly split is going to be SSS, and as such, subject to massive swings in variance with minimal predictive value. The best hitter in August (min 50 PA) was Joaquin Arias, after all. Trying to ascertain anything from samples of 50-100 plate appearances is a fool’s errand.

However, on the second point, I’ve never actually seen anyone show that offense in September spikes up dramatically after rosters expand. Intuitively, it makes some sense that more bad pitchers should lead to more runs, but at the same time, more pitchers available leads to more match-up opportunities and more aggressive bullpen usage, which should lead to fewer runs scored. So, I decided to look at the last 10 years, and see whether the sport saw any kind of noticeable change in September offense.

Season	AVG	OBP	SLG	wOBA
2003	0.264	0.333	0.422	0.328
Sept.	0.262	0.330	0.416	0.325
2004	0.266	0.335	0.428	0.330
Sept.	0.261	0.330	0.419	0.326
2005	0.264	0.330	0.419	0.326
Sept.	0.261	0.330	0.413	0.324
2006	0.269	0.337	0.432	0.332
Sept.	0.265	0.332	0.427	0.328
2007	0.268	0.336	0.423	0.331
Sept.	0.274	0.343	0.435	0.339
2008	0.264	0.333	0.416	0.328
Sept.	0.266	0.336	0.420	0.331
2009	0.262	0.333	0.418	0.329
Sept.	0.262	0.333	0.408	0.325
2010	0.257	0.325	0.403	0.321
Sept.	0.250	0.319	0.390	0.314
2011	0.255	0.321	0.399	0.316
Sept.	0.255	0.322	0.405	0.319
2012	0.255	0.319	0.405	0.315
Sept.	0.254	0.320	0.398	0.314

In seven of the last 10 seasons, league wOBA in September has been lower than the season average overall, this year included. There were jumps in both 2007 and 2008, but those were followed by equally large declines in 2009 and 2010, so it’s hard to say that there’s any kind of trend toward increasing offense in the final month of the season.

It’s possible that looking at league wide numbers is obscuring a real effect, and some guys are generating a real benefit from facing pitchers in September that they wouldn’t get to face earlier in the season. However, for every team that is out of the race and giving experimental looks to guys called up from the minors, there is another team that is desperately trying to win every game they play, and now has the advantage of a deep bullpen of situational arms with which to play the match-ups. Over a month’s worth of playing time, these things probably don’t even out for every hitter in baseball, but there simply aren’t enough scrubs throwing lots of innings around the sport to substantially alter the run environment in the final month of the season.

For the guys closing out bad seasons by finishing strong, we should be skeptical of their performance because it was relegated to a single good month, but we probably shouldn’t be any extra skeptical of that performance simply because it came at the same time as the rosters expanded. There might be a correlation between larger rosters and the timing of when these guys got hot, but that doesn’t mean that the larger rosters caused the hot streaks.