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September 3, 2007

Lies, Damned Lies

The Contenders' Rotations

by Nate Silver

I’ve done radio gigs in several different markets over the course of the past couple of weeks, and very often the first question I get asked is about the perceived inadequacy of the home team’s starting rotation. Sometimes—as in the case of the Mets—the question is valid, but nobody is entirely happy with their pitching staff this time of year. Someone is always injured, or slumping. Maybe a team has a deep rotation but no ace, or maybe it's the other way around.

To help sort out the answers, I have prepared a handy guide to the starting rotations of the 16 teams that retain a tangible shot of reaching the playoffs according to the latest version of the playoff odds report. The metric I’ll be using is QERA, which evaluates a pitchers performance based solely on his strikeout rate, walk rate, and groundball-to-flyball ratio, three numbers that stabilize relatively quickly, and are largely unaffected by park and league effects. For a more detailed guide to QERA, please read last year’s piece on playoff hurlers.

Teams are rated according to both their September and October QERAs. “September” represents performance over the balance of the regular season, assigning full credit to the #1-4 starters, and half-credit to the #5 starter, who will get skipped as often as possible this time of year. “October” represents what happens if a team is fortunate enough to reach the playoffs, where the rotation becomes significantly more top-heavy. It assigns 31 percent of the credit to a team’s #1 starter, 28 percent to the #2, 23 percent to the #3, 18 percent to the #4, and no credit at all to the #5 starter, based on an empirical accounting of how teams structure their rotations in the playoffs. I’ve then ranked the teams from 1 to 16 with an eye toward both September and October performance, making a few judgment calls along the way in cases where I think that QERA might not be telling the whole story.

Just a little bit more housekeeping and then we’ll get underway. I have assigned a slight penalty to National League pitchers, reducing their strikeouts by five percent, which should roughly cancel the boost they get by getting to throw to the opposing pitcher (Colorado pitchers were exempted from this penalty, since that is the one park that does tangibly reduce strikeouts). Also, you will see some pitchers with an asterisk (*) next to their name, which means that their QERA incorporates minor league statistics. In these cases, I have evaluated their QERAs at the minor league level, and then added 1.25 runs to represent the rough skill difference between the majors and advanced minors. Thus, Ian Kennedy’s 3.10 minor league QERA becomes a 4.35 major league QERA, or EqQERA if you prefer.

16. Rockies

Francis    3.94
Cook       4.63
Jimenez*   5.33
Fogg       5.02
Morales*   5.74
September  4.84
October    4.65