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.









(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:







