Posted by Alex Bonilla on August 31, 2021
Today we released an update to how we calculate one-year park factors for 2020 and three-year park factors that include 2020. In short, we are giving the observed effects of ballparks in 2020 less weight, impacting context-adjusted stats like ERA+, OPS+, Rbat+, and WAR for 2019 through 2021.
There are two reasons for this change. First, the shortened 60-game season decreases the sample of games we have data from, which naturally reduces the significance of the data collected. Second, since teams only played within their own divisions in 2020, comparing scoring in home games vs. scoring in away games does not tell an accurate story of how a park impacted scoring relative to league average, since most of the parks in the league are not included in either set of games (e.g. when computing the park factor for Wrigley Field, games played at Coors Field or Citizens Bank Park are not included in the calculation anywhere, since the Cubs did not play away games at those parks in 2020).
The issue with the way we had been handling 2020 park factors became more apparent as the 2021 season went on, particularly because the Cincinnati Reds’ 2020 park factor of 119 was raising the 2021 three-year park factor, resulting in worse-than-expected adjusted stats for hitters like Joey Votto, and better-than-expected adjusted stats for pitchers like Wade Miley.
Now, when you look at a 2020 team page, the one-year park factors have been diluted so that they include an average of 60 games’ worth of 2020 data, and 51 games each of 2019 and 2021 data. If there is no corresponding 2019 or 2021 data (e.g. new ballpark in Texas, different mix of parks for Toronto), those parts are replaced with a league-average park factor of 100. These new one-year park factors are used in the three-year averages like usual, so the effect is reflected there as well.
With this change, here are some of the most notable movers in Wins Above Replacement:
Zack Wheeler (+0.5) and Aaron Nola (+0.3) each saw a bump up in their 2021 pitching WAR as the 3-year park factor for Philadelphia rose from 96 to 98 (frequent opponent Washington also had their 3-year park factor increase from 93 to 96). Wheeler’s 0.5 is the largest change resulting from this update.
Wade Miley, Tyler Mahle, and Luis Castillo (-0.4 each) saw their 2021 pitching WAR fall. As mentioned above, Cincinnati saw some of the most anomalous park factors in 2020, and mitigating their impact here lowers the expected run environment for these and other Reds pitchers.
In 2021, Shohei Ohtani is notably untouched by this update, with changes to his batting WAR and pitching WAR canceling each other out and his 7.9 total WAR remaining the same.
Here is a full list of changes to park factors, rate stats, and Wins Above Replacement from before and after this change.