Run Differential, the Neyer Way

Here's the garbage. Here's the garbage man:

"NFaleris (5/11/2007 at 4:28 PM)

I think the bigger issue is that run differential is better used as an analytical tool (e.g. for isolating "luck") than as a predictive tool. Not only will blowouts (like the A's and Rangers yesterday) affect the predictive capacity, it will have an ABSURD effect this early in the season. While I agree with most of what you write, Rob, it's a little silly to think the run differential after the 24th game of the season would tell us the Cubs would be a potential team to beat -- which is essentially what you say above."

(temp link, look for "Run differential Matters" Rob Neyer May 11th, 2007)

To add to NFaleris's great comment, the Blue Jays had a +23 on that date as well, and were 13-12. Now they're 15-21 with a minus 20 differential. Run differential is analytical to see how a team is performing, not necessarily predictive without a huge sample (but then again what isn't predictive after a huge sample?). That's what I was getting at back in "The Illusions of April." I should probably email that blog to Rob again, I don't think he read it too closely.

Personally if I try to predict a hitter's future performance, I look at OBP - AVG, and SLG. If the guy has OBP - AVG at 0.07 or higher I predict good things, same for SLG over .500. For pitchers I look at K/BB only. About 3/1 ratio and higher I predict good things. This baseline is pretty much based empirically on Webb's past performance, since he's the most extreme contact pitcher who is actually good, and that's about what his K/BB is generally. I think the same stats are good predictors for teams, but also record in 1 run games is relevant since a team with a very good 1 run game record will most likely fall off their pace and vice versa. In this way you can avoid the skewed data of run differential where a couple blowups from fringe pitchers can impact it.

Caveat: Some weird things happen when players get old. Old hitters often increase their walk rate as their bats slow and they foul more pitches off instead of putting them in play. This brings up their OBP - AVG. Since higher OBP helps their team it's still not a bad predictor of "good things." But on the flip side, as great pitchers age, they can maintain a high K/BB as they lose effectiveness. I hypothesize that this is because "stuff" degrades before control, so a guy could continue throwing strikes as his stuff gets worse, leading to more hits and runs scored against. That's not a "good thing", but anecdotally it only seems to happen to the great ones who refuse to paint the corners even as their stuff diminishes, since they've never pitched that way before. See Schilling (2005) who seems to have adapted or Johnson (2006-) to see what I'm hypothesizing about.