The humidor at Chase Field: 70 games in

Game 70 was last night’s win over the Braves, so it’s time to check in once more and see if we can figure out the impact of the humidor this season. Anecdotally, either we seem to be getting used to it, or it’s settling in, because I don’t recall reading quite so many “#humidored” comments of late.

Runs scored and allowed

A quiet spell. The biggest tally was the 13 runs in the opener of the Atlanta series, and that was the only time double digits were reached. Overall, in the last ten games, the Diamondbacks scored 46 runs and conceded 33, for a total of 79 games. That was down a chunk on the same period in 2017, when they scored 57 and conceded 42, making the total twenty runs more, at 99. We haven’t seen a bigger deficit than this -20 figure since the very start of the season, when Humipocalypse took place, with a -47 change over the first ten games at Chase.

The overall difference this season is now ninety below what it was last year. However, almost all of that difference came in the first thirty games of the season, when the deficit sat at -84, with only six runs being “lost” since that point. Interesting that the drop has not been an even-handed on. The D-backs have scored 73 runs fewer than the first seventy games at Chase last season, but conceded 17 less. But it’s difficult to be sure how much of that is due to the humidor and how much, for example, to the team’s offense being worse e.g. no J.D. Martinez to boost things down the stretch run.

Here’s the chart.

Overall offense

This had an impact on Chase Field ERA, which dropped four points on the season to 3.88, and increased the gap between it and NL average ERA to fifteen points (4.03). I thought it might be interesting to see whether there was any difference between left- and right-handed hitters. In 2017, Chase was a very even park, with only one point of OPS difference across the split. A slightly-higher OBP for left-handed hitters, was basically negated by a lower slugging percentage. Would that still be the case in 2018?

• 2017 RHB: .252/.323/.453 = .776 OPS
• 2018 RHB: .237/.310/.394 = .704 OPS
• 2017 LHB: .263/.335/.440 = .775 OPS
• 2018 LHB: .248/.323/.398 = .721 OPS

We’re talking about 3,500 PA for right-handers, and 1,800 for left-handers, so are out of the realm of small sample size to this point. It initially appears that right-handed hitters may have been affected more, their OPS dropping by a third more than the change for left-handed hitters. The change in that gap all comes from slugging. The split in OBP between the two groups has barely changed, both sides dropping by about a dozen points. But the slugging edge right-handers enjoyed in 2017 has gone, with left-handers now showing slightly more power. However, this is a regression to normal. Over the whole pre-humidor era, LHB had an OPS 28 points higher overall (.782 vs. 754) at Chase.

Home/road splits

A quick refresher. Over the first twenty years of the franchise, the team has always hit better at Chase Field than elsewhere. The amount has varied, from a mere three OPS points in 1999, to 136 points last year, but it has been a consistent feature for two decades. Of course, run environments have shifted dramatically in that time, as we went through the steroid era and came out the other side. But it’s the difference which we’re interested in, and from 1998 through 2017, here’s the sum total of the Diamondbacks home/road offense:

• Home: .269/.338/.442 = .781 OPS
• Away: .248/.315/.394 = .709 OPS

Now, let’s look at those same figures for the Diamondbacks’ performance at the plate in the 2018 season. We’ve played 70 games at home and 71 on the road:

• Home: .248/.328/.401 = .729 OPS
• Road: .230/.302/.401 = .702 OPS

The split has widened quite a bit since we last checked in, going from 18 OPS points to 27 points. Curiously, in both cases, all the change comes from on-base percentage, with slugging percentage unchanged both at home or on the road. At Chase Field, OBP has increased by four points, but it has dropped by five away from Phoenix. The hellish road-trip through San Francisco and Los Angeles, where the Diamondbacks scored twelve runs in seven games, and batted .183/.242/.301 for an OPS of just .543, is clearly a significant contributor. But at +27 OPS points, I’d say the overall humidor impact has likely been close to what was wanted.

A final check in will occur after the season is over.