The humidor at Chase Field: 20 games in

Last night was the 20th game played at Chase Field since the introduction of the humidor, with the two sites combining for three runs over 11 innings - and one of those, scored on a balk. We’re still early on, relatively speaking, and the D-backs offense has been struggling to score runs of late, almost regardless of the location. But we now have double the amount of data compared to when we took our first look at the situation. So let’s see whether the trends identified over the first ten games with the humidor in effect, have held up over the next 10 games. We’ll look at the same metrics as we did then.

Runs scored and allowed

Through 10 games, the D-backs had scored 43 runs and conceded 29, both figures dropping around 40% when compared to the same period last year. In games 11-20, we saw 66 runs being scored, evenly divided between Arizona and their opponents. That makes the tally to this point 76-62, compared to after twenty games last year, when the score was 122-83. That’s a drop of 36% in runs scored and 25% allowed; overall, we have had 138 men cross home-plate in 2018, compared to 205 the previous year, a drop of 32.7% in total scoring. So the gap has narrowed somewhat, but we’re still down about one-third.

Overall offense

It is worth noting that scoring is down all over baseball this year. But ERA is lower MLB-wide by only about one-quarter of a run. At Chase, the drop has been an awful lot more, going from 4.70 last season - and that with one of the best pitching staffs in baseball calling it their home field - to 3.20. That’s a full run and a half drop through the first twenty games. It’s currently ranked 28th in park ERA among the 30 major-league stadia, compared to 7th in 2017. So far, the indications are that the humidor has not made the park neutral, but turned it into a significant pitcher’s park.

Home-runs continue to be particularly hard to come by. There have been 35 at Chase this year, one every 43.1 plate-appearances, a rate almost 50% higher than the figure in 2017 (28.9). But hits of any kind have been notable by their absence. All hitters at Chase combine for an average of only .215, which is the worst in the majors [I guess Nick Ahmed, at .218, is now an above-average hitter...]. Last time, we noted the extraordinary drop in BABIP. That has regressed as we suspected it might - but only somewhat. By six points, in fact. Batting average on balls in play at Chase Field is still .268, which is 29th in the majors and thirteen points below where any park played over the whole 2017 season.

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 20 games at home and 17 on the road:
Home: .223/.311/.373 = .684 OPS
Away: .233/.308/.429 = .737 OPS
The difference has swung from an average of +72 OPS points before this season, to -53 this year. It’s a sharp increase from the -8 we say after ten games, mostly because the Diamondbacks have been hitting a lot better on the road. Their OPS there increased from .701 the last time we checked in. But their road BABIP is still ten points lower than the home one, so the gap doesn’t seem to be particularly attributable to luck. Additionally, Arizona has yet to make a visit to Coors Field, a trip which can typically be expected to inflate those road offensive numbers some more.

It doesn’t seem to be affecting every player equally, though we are likely still too small in the sample size to draw any real conclusions. With that taken as read, A.J. Pollock seems most unaffected, with six home-runs and a 1.069 OPS so far. At the other end, the biggest victim might be Paul Goldschmict, who is batting a mere .138, and we’re still waiting for his first home-run at Chase Field, 85 plate-appearances into the season. But it’s hard to tell how much of that is the humidor, but Goldy’s road OPS is a perfectly healthy .910, almost exactly at his career figure (.904). Paul’s slump is entirely the result of his performance at Chase Field, where his OPS is 372 points below his career norm of 944.

A good chunk of that is BABIP related: at home, Goldschmidt’s BABIP is only .237, compared to .372 elsewhere, which suggests either a huge difference in luck, or that he’s making far better contact on the road, with more barrels. It would certainly be a stretch to say that the humidor is the cause, but the difference is striking, and provides food for thought, perhaps worthy of further investigation. Something to keep an eye on when we check back next, after home game #30 against the Marlins on the first day of June.