The simple answer to the question in the title is YES, there is precedent for large drops in offense and the components of offense at Chase Field prior to 2018.
The traditional formula for basic park factors measures what a team does at home and on the road, against what they allow to their opponents in home and road games. The formula looks like this:
((homeRS + homeRA)/(homeG)) / ((roadRS + roadRA)/(roadG))
This is the most simplistic form, and what you will find at ESPN Stats pages. These are single year park factors. Remember, 100 = neutral, the more over 100, the more hitter friendly, the more under 100, the more pitcher friendly. Also, while Chase dimensions have not changed and for the most part games are played indoors, the league around them DOES change. These numbers are all relative to the league. So while Chase itself may not be changing, how it plays relative to the other stadiums in the league DOES change.
Baseball-Reference uses a more complex set of calculations, and uses a multi year weighting system, which you can read about here. (Warning, NSFMA, or Not suitable for Mathaphobes). Fangraphs also has park factors, but they don’t show 2018 and they don’t show their calculation method. However, for the very specific purpose of this post, I am using the simple one year park factor calculation to illustrate a point:
SUDDEN DROPS IN CHASE PARK FACTOR HAPPENED THREE TIMES IN LAST 10 YEARS.
Lets look first at Homeruns. Below you can see clearly the HR hit and allowed by AZ hitters and pitchers at Chase, against what the AZ Hitters and Pitchers and their opponents produced in DBacks road games. A you can see, the HR factor at Chase showed precipitous drops in 2013, 2015, and of course 2018.
But of course offense isn’t only about Homeruns. Below table shows the park factors for 10 years over each major component, and also wOBA, which includes everything. The range in wOBA is tighter than the range in runs, which is interesting. The color coding is simply to help highlight how big the drops were in 2013, 2015, and 2018 compared to previous seasons. Chase is always a triples friendly park, but the swings are large, due to sample size issues. Hits and Doubles more or less track with Runs and wOBA, (except 2015 doubles), and Chase has been surprisingly walk friendly the last 4 years compared to the previous 6.
Now at this point I think we also need to take a look at the Baseball-Reference single year and multi year park factors. Because we reference OPS+ and ERA+ a lot and of course bWAR, it’s important to note that BR uses Multi Year park factors to calculate those. However because the multi year park factor uses 3 seasons, the season immediately before and immediately after the season you are measuring, it creates an obvious problem. 2019 hasn’t happened yet. So the “multi year” park factor for 2018 is really only 2017-2018 combined at the moment. As the 2019 season progresses and is later completed, it will actually affect the multi year park factor for 2018, which will in turn retroactively alter the 2018 OPS+ and ERA+ and WAR numbers, (everything that is subject to park adjustments) . Depending on how you feel about things, this can be both a feature and a bug. The underlying premise is that the park factors are not precise enough to rely on one year of data. However BR determined the most accurate way to measure is using the season before and after. Which means we need to actually wait until the following season (in this case 2019) is complete before we can take a truly final look at 2018. It can be confusing. So what the hell am I talking about ? Look at the table below first, which are the BR Single and Multi year factors:
As you can see here, while the single year factor for 2018 dropped quite a bit from 2016-17 levels, they are roughly the same as 2013 and 2015, the previous years of “lower offense” generated at Chase. But the MULTI year didn’t really drop much at all because it’s only using 2017-2018 data. HOWEVER.....if the humidor effect was real, and therefore offense stays down in 2019 and does not pop back up like it has in the past, then we will have a greatly reduced multi year park factor for 2018. And if that happens, you will see the 2018 hitters OPS+ and WAR Rise, and the pitchers ERA+ and WAR decrease, all retroactively. If this were to occur, it would actually give us a more accurate view of 2018 performance........just delayed.
Clearly offense factors relative to the rest of the league were lower in Chase field in 2018 compared to most seasons. But this has happened several times before in the recent past. So while most pundits will attribute the drop in offense factors to the humidor, we actually need to see at least one more consecutive season of this reduced level of offense at Chase to be able to say with any certainty that it is due to the humidor.