Wow, as someone who claims to be one of the nation’s leading Brian Bannister scholars, I am embarrassed to say I missed this. But I did. It is brilliant reader PC who points this out:
Brian Bannister by day (this year):
– 4-0, 0.62 ERA, 29 ip, 12 hits, 3 runs, 2 earned runs, 0 homers, 7 walks, 18 K.
Batting average against: .126; OPS: .320; OPS+ -10(!)*, Babip: .156.
Brian Bannister by night (this year):
– 0-4, 8.02 ERA, 21 1/2 ip, 33 hits, 19 runs, 19 earned, 5 homers, 5 walks, 11 K,
Batting average against: .344; OPS .960; OPS+ 158(!), Babip: .350.
Brian Bannister by day (career)
– 10-1, 2.65 ERA, 88 1/3 ip, 67 hits, 28 runs, 26 er, 4 homers, 23 walks, 38 Ks.
Batting average against: .212; OPS: .576; Babip .226.
Brian Bannister by night (career)
– 8-13, 4.58 ERA, 165 ip; 168 hits, 88 runs, 84 er, 20 homers, 55 walks, 87 Ks.
Batting average against: .261; OPS .763; Babip: .275.
Those are pretty dramatic splits, certainly. But I think it's dangerous to read too much into them. Yes, this year Bannister has been much, much, MUCH better in the daytime than under the lights. But you can't say it's not a fluke simply because his career numbers also reveal a split - because those career numbers include this season.
For his career, Bannister has a 4.58 ERA at night, a 2.65 ERA during the day. But if you strip out 2008, his ERA at night is 4.07; during the day it's 3.64. A difference, but a small difference. And it's a difference which overstates the mark, if anything. Here are Bannister's career totals at night and during the day, prior to 2008:
Night: 143.2 IP, 135 H, 47 BB, 78 K, 15 HR
Day: 59.1 IP, 55 H, 16 BB, 20 K, 4 HR
His hits per nine innings during the day (8.34) is a fraction better than his ratio at night (8.46). His strikeout-to-walk ratio is significantly better at night than during the day. His BABIP - this is approximate - is .256 during the day, .266 at night.
Prior to 2008, there was no conclusive evidence to speak of that Bannister was a better pitcher during the day than at night. Do eight starts change that perception? If his start in Arlington, with 30 mph winds gusting to right field, when Bannister said he felt like he was pitching on the moon, had happened to occur in the day, how much would that skew these numbers?
I do think that Bannister is probably a little more effective during the day, in part because Bill James ran a study about 20 years ago - I think it was the 1987 or 1988 Abstract - which showed that power pitchers appear to be signficantly more effective at night. Bannister is basically the opposite of a power pitcher, so it would stand to reason that he might be more effective during the day.
But I don't think the difference is great enough to influence how the Royals use him. For one thing, he's not the only Royals' starter to pitch better during the day. Gil Meche, in a career of over 1000 innings, has a daytime ERA (3.78) more than a point lower than his nighttime ERA (4.84). Now that is significant. I suspect that most pitchers pitch better during the day, because day games tend to be clustered in the colder months of the season, and colder weather tends to lower offense. (Just a theory I have. Which means I'm probably wrong.)
If the Royals have a doubleheader and choose to start Bannister in the day game and someone like Greinke (whose ERA is 15 points under the lights) in the nightcap, great. But let's not go overboard with moving starters around to take advantage of an effect that might not actually exist.