Today’s New York Times has an interesting article that analyzes the performance of pitchers in the National and American Leagues, and leads to the conclusion that most pitchers are better off in the Senior Circuit:
For all its status as the senior circuit, the National League has become a shelter of sorts for pitchers, a safe haven from A.L. lineups that tend to pelt them with three-run homers and force early showers. Like swinging two bats in the on-deck circle, pitching in the A.L. ? with its designated hitter and far stronger No. 9 batters ? makes moving to the N.L. feel downright liberating.
Teams and fans have long understood that a pitcher can see his E.R.A. fluctuate by half a run just by switching leagues, as if stepping up to or down from a high curb. But evidence suggests that, less visibly, a pitcher?s relative effectiveness ? his performance compared with that of his league peers ? is affected as well.
From 2000 through 2005, 57 starting pitchers (those with at least 20 starts that season) switched leagues the next year ? 29 to the N.L. from the A.L. and 28 in the other direction. Their statistics moved with them: Combined E.R.A.s for the new National Leaguers decreased to 3.94 from 4.79, or 0.85 of a run, while their counterparts? increased to 4.64 from 3.94, a move of 0.70.
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A statistic called E.R.A.+, presented on the Web site baseball-reference.com, adjusts for these influences and presents a pitcher?s percentage, either above or below a league?s average. For example, Zito?s 3.83 E.R.A. last season in Oakland ? a good pitchers? environment ? translates to a figure of 116, or 16 percent better than the A.L. average.
Theoretically, a pitcher?s E.R.A.+ should not be affected much by a change in leagues. But switching circuits still seems to make a substantial difference in how a pitcher performs.
Of the 29 pitchers moving to the N.L. from the A.L., their E.R.A.+ figures increased to 110 (10 percent above league average) from 97 (just below average). This smaller shift than in E.R.A. is nonetheless more significant: It indicates that starters of equal caliber are more successful in the less suffocating National League.
(…)
Pitchers found moving to the A.L. from the N.L. correspondingly unpleasant ? the E.R.A.+ scores of the 28 pitchers decreased to 100 from 113, or to absolute average from healthily above. (The fact that the two groups moved 13 percentage points in opposite directions was purely coincidental.) A fair interpretation, then, is that moving to the A.L. is such a challenge that pitchers, at least temporarily, regress. Take the case of Boston?s Josh Beckett, whose 5.01 E.R.A. rose faster than the homers he allowed.
This, perhaps, accounts for the stunning success that Roger Clemens, an older pitcher who was becoming less effective at the A.L. Yankees, had one of the best seasons of his entire career in his first year at the N.L. Astros. What it means if he returns to New York, of course, remains to be seen.
