From FanGraphs, when pitching statistics become reliable:
150 BF – K/PA, grounder rate, line drive rate
200 BF – flyball rate, GB/FB
500 BF – K/BB, pop up rate
550 BF – BB/PA
In case it’s not obvious, you can tell a lot more about a hitter from one year of data than you can about a pitcher. All this data is from research that Pizza Cutter conducted, which can be found in the links below. If a statistic is not included, the means it did not stabilize over the intervals that Pizza Cutter tested (which was up to 750 PA / BF).
Last year, Cole Hamels faced 26 batters per game on average, which means that we need to see at least six starts (about one month) before we can begin making up Hamels-related theories.
Hamels was bad last night. Was it a fluke? Maybe. Is his performance a bad omen? Maybe. In one isolated game last night, Hamels had trouble with his command (two walks in two and two-thirds innings) and was a bit unlucky on balls in play (lots of bloop hits, a scorching line drive by Ike Davis notwithstanding). Perhaps the weather bothered him, though he pitched just fine in the cold, rainy weather during the 2008 World Series.
I can see the Bleacher Report articles now: “10 reasons why Hamels doesn’t have the mental fortitude to pitch in April” or “17 signs Hamels is offended to be the #4 starter”. The 610 WIP callers will scream about how Hamels has become complacent pitching behind three aces.
Using sample size constraints limits discussion topics, but you’ll thank yourself at the end of the season when you think back on the season and don’t look like an idiot for projecting doomsday scenarios, as many did with Hamels last year. I get it, though. Corey Seidman said it beautifully at Brotherly Glove:
[...] [we] don’t hate Hamels, [we're] just so much more passionate about him because he dated us before we had money.