Wednesday, June 17, 2020
Park Impact, 3 of N: Homeruns
In part 2, we looked at strikeouts. If you go in the comments of that thread, I added the walks, which was fairly uninteresting. Now, the homeruns. As a reminder, or maybe I never mentioned it, I am using the Quick way. That means looking at batters at home and away, as well as pitchers at home and away. And "home" is the team's main home. If teams play at a neutral-site game, neither is home. And if a hurricane causes a team to host another team, but the host team bats first, the host team is considered home.
(Click to embiggen)
So, for those who haven't followed the series: this shows how many HR are added at that park, per plate appearance. Coors adds 0.006 HR/PA, while Oracle Park subtracts 0.007 HR/PA (relative to the away parks). More red, more HR, more blue, less HR.
The last column shows the standard deviation of the seasonal park impact numbers. The mean is 0.003 (or 0.0031 for more precision). What would we expect due to Random Variation (not even including wind, temperature, and park changes)? That would be 0.003 (or 0.0030 for more precision). In other words, the season to season changes you see are what we'd expect from Random Variation. I have to say, I am surprised. It's more likely that this Quick method is just not good enough. I'll do it the good or better way soon. Still, given the results I see, I doubt we'll see anything that will be noticeably different. I mean, I might have expected 0.0035, maybe as high as 0.0040. Which means that visually, it won't stand out any different.
Anyway, given that, it's far more likely that we should use a standard park impact number, rather than a seasonal one. If you want to argue for a rolling five-year, that's probably fine. We should make adjustments by temperature, though that will only be felt at the game level, and hardly make a dent at the seasonal level. And if you are aware of park configuration changes, like Citi went through some years back, you should use that too. So, HR park impact numbers, more than any of them, require some SME (subject matter expertise) hand-holding.
And here's how it looks comparing HR to wOBA. As you'd expect, some strong relationship.
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