The humidor at Chase Field: 10 games in

Last night marked the 10th game played at Chase Field this year, and so the 10th game played with baseballs which had been stored in the humidor. While it’s still obvious very early days, we now have 765 plate-appearances in Phoenix to use as data points, and following up on shoe’s fanpost from a couple of days ago, I thought it might be worthwhile to see what that shows us. Obvious caveats apply: there are a range of other factors playing into the results, which can’t be controlled for. These include the opponent, the Diamondbacks’ roster, etc. To help moderate that, I want to look at a range of metrics, and not just take (say) OPS.

Runs scored and allowed

This is the bluntest and broadest measure of offense, but is perhaps also the one which has the most dramatic effect. Through ten games at Chase Field last year, the Diamondbacks had scored 74 runs and conceded 45. Through the same number this season, the Diamondbacks had scored 43 and conceded 29. That’s a 42% drop in RS, 36% in RA. Overall, the total number crossing home-plate in Phoenix has declined year-on-year from 119 to 72, a 39.5% drop. The chart below shows the cumulative number of runs thus far. After the first four games almost kept pace, scoring since has been consistently below the corresponding contest in 2017.

Overall offense

Across those 765 plate appearances, by all hitters at Chase Field, the overall line has been .211/.287/.356, an OPS of just .643. Only Busch Stadium (.641) has seen a lower OPS among National League parks this year. It stands in sharp contrast to last season, when the line at Chase was .256/.327/.449, a .776 OPS that was third in the NL, behind Coors and Great American Ball Park. Part of this is the obviously expected result: a decrease in homers. In 2017, we saw a home-run here once ever 28.9 PAs. In 2018, that figure has been one in every 38.2 PA. As a yard-stick, last season, Petco Park had a home-run per 32.6 PA, and Marlins Park 35.4. So far, Chase has played less HR-friendly than those seaside stadia.

But it hasn’t just been the balls going over the fence. Those which stay in the park have also been turning into outs with greater frequency. In 2017, balls in play at Chase Field had a batting average of .309, also third in the National League. This season, that has cratered by 47 points to .262, ranking our park 14th. Now, this figure does seems highly likely to regress over time: last season, no park in the NL saw a BABIP of below .281. But it does appear that we may be seeing more balls which would have gone off the wall last season, dropping harmlessly into outfielder’s gloves. It’s possible infielders are also vacuuming up those squishier, slower groundballs, which might have scooted past them in 2017.

Home/road splits

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

This season? If the trend to this point holds, it will be the first year ever where Arizona has hit better away from Chase:
Home: .225/.315/.378 = .693 OPS
Away: .217/.287/.413 = .701 OPS
Yes, it’s only eight points difference - that’s still an eighty-point swing from the average of the previous twenty years. And there are a couple of reasons we might expect the road offense to pick up and widen the gap. We’ve played so far in Busch Stadium, AT&T Park and Dodger Stadium, and not yet gone to Coors. The road figure above is likely suppressed too, by a BABIP of .269, twenty-three points lower than Arizona’s same number in Phoenix thus far. [While BABIP has historically been higher at Chase for the team, .307 vs. .288, as mentioned above, we may well be looking at a significant drop in the figure here]

It’s also interesting to note where the OPS difference comes from. The D-backs’ on-base percentage is still better at Chase, by very close to the previous margin - +28 vs. +23 points. But the difference in the team’s SLG has been flipped on its head, and gone from +48 to -35. This suggests it’s the decline in home-runs and extra-base hits which are responsible for the overall difference.

We’ll keep an eye on all this going forward, and see whether these very early trends end up being sustained. The 20th home game will be the opening contest against Washington on May 10th, so that seems a good point to revisit the above metrics and see what’s going on with them.