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Last year, I watched a lot of Orioles day games, mostly on Sundays. There are few things more fun than waking up, eating breakfast, drinking some coffee, and settling in to watch a ballgame on my sofa in my air-conditioned house — particularly during a humid Texas summer. This is why, in 2014, I wrote a lot of recaps for Sunday games.
Problem is, I recapped a lot of losing games. It was very frustrating. But looking back, it’s not surprising: the Orioles had a wOBA of .310 in day games last year, 19 points worse than their night-time wOBA of .329. That’s a lot of offense to "lose" over the course of a season. In fact, since the average team had a wOBA of .310 during the day last year, you can say the Orioles were an average offensive team when the sun was shining. At night though, they turned into Mr. Hyde: their .329 wOBA was far and away better than the major-league average of .312.
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(Note the y-axis scale that starts at .300 wOBA.)
So when I recapped my first game this year, a losing effort on Sunday April 12th, I wondered if this was a trend I should be worried about. Day games are generally after night games, and so players may be more tired after having played the night before. The manager may rest players or yank them earlier than normal. Are fans doomed to watch the Orioles lose during the day, and am I doomed to never post a "Most Birdland Player" poll on Camden Chat?
There’s a couple things to unpack here. First, as with their wOBA split in 2014, some teams absolutely hit worse during the day than at night. Even over the long run, this is true. I studied the 20 years from 1995-2014 and found that daytime wOBA varied from night-time wOBA by as much as 10 points in either direction. The worst is the Texas Rangers, who lost 10 points of wOBA during the day. The best is the Chicago Cubs, who gain 8 points of wOBA during the day. (The Cubs play far more day games than any other team.)
But a few things convince me that fans shouldn’t worry about 2014’s daytime performance affecting 2015’s. First, although the Orioles hit worse during the daytime, they still scored plenty of runs: 4.35 runs per game overall in 2014, 6th-best in the AL. Second, notice that their 2014 difference of 19 points was well beyond the 20-year observed maximum of 10 points. This indicates that teams’ night/day wOBA differences regress to the mean. The average team in the above sample lost only 1 point of wOBA during the day.
That difference was so small that it seemed meaningless. I had a hunch that teams who hit poorly during the day also hit poorly at night, and vice versa. To find out, I ran a correlation of night-time wOBA against daytime wOBA and got the following scatterplot:
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That's a pretty nice-looking plot. The regression equation is Daytime wOBA = .829 * Night-time wOBA + .056. If you plug in the Orioles’ 2014 night-time wOBA of .329, you get an expected daytime wOBA of .328. Their actual 2014 daytime wOBA of .310 was an extreme underperformance! Fans should have expected them to lose 1 point of wOBA when the birds (pardon the pun) were chirping, but they lost 19 points instead.
What could have caused that underperformance? The r-squared in the sample above is .716, indicating night-time wOBA explains nearly 72% of the variance in daytime wOBA. The variance in this case is infinitesimal: .00002373 wOBA. That’s not a typo; there’s just not much variation to be had.
Combined, the r-squared and the variance tell us a lot. First, there's not a lot of separation between teams’ wOBA in the daytime. Second, what little variation there is can be explained more than adequately by their wOBA at night. This means there isn’t much of a secret sauce to explaining why a team may have under- or over-performed their expected daytime wOBA.
This is where sabermetricians bring in the word "luck" as a shorthand for "things that happened but don’t happen consistently enough to be more predictable than randomness". Some examples of what could’ve lowered the Orioles’ 2014 daytime wOBA: having to face good pitchers during the day, an Oriole hitter missing the ball by a fraction of an inch, the opposing pitcher putting a touch more spin on their curveball, the wind turning a home run or double into an out, a defender making a great play on a well-struck ball, etc.
There’s evidence to indicate it was these types of events, not an inherent defect in the 2014 Orioles, that caused the 2014 team to hit poorly during the day vs. at night. And there’s evidence to suggest these events typically account for 28% of the variance in a team’s daytime wOBA. Perhaps there are underlying causes that can be explored in a future study, but as the very small variance shows, such a study is not likely to have much of an effect.
For now, fans shouldn’t expect the 2015 Orioles to underperform as badly as they did last year. They should expect something akin to their overall wOBA of .340 (as of this writing), and I’ll continue to watch daytime Orioles baseball and hope for the best.