Baseball Analysis | Switching to NL tends to help pitchers

The quip has become a chorus.

In debating whether Barry Zito will earn his $126 million for seven years from the San Francisco Giants after posting a combined 3.85 earned-run average with Oakland the last two seasons, many observers say, "But he is going to the National League."

Roger Clemens is considering a return to the New York Yankees or the Boston Red Sox after steamrolling opponents in Houston for three seasons, and warnings sound, "But he'll be moving back to the American League."

Randy Johnson gets himself traded from the Yankees back to the Arizona Diamondbacks, and everyone expects his ERA (5.00 last season) to plummet: "He'll be much better back in the National League."

For all its status as the senior circuit, the National League has become a shelter of sorts for pitchers, a safe haven from AL lineups that tend to pelt them with three-run homers and force early showers. Pitching in the AL — with its designated hitter and stronger No. 9 batters — makes moving to the NL feel liberating.

Teams and fans have long understood a pitcher can see his ERA fluctuate by half a run merely by switching leagues, as if stepping up to or down from a high curb. But evidence suggests, 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 NL from the AL and 28 in the other direction. Their statistics moved with them: Combined ERAs 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.

ERA is shaped by more than a pitcher's talent: His league, home-ballpark dimensions and other factors can greatly distort an ERA and its interpretation.

A statistic called ERA-Plus, presented on baseball-reference.com, adjusts for these influences and yields a pitcher's percentage, either above or below a league's average. For example, Zito's 3.83 ERA last season in Oakland — a good pitchers' environment — translates to a figure of 116, or 16 percent better than the AL average.

Theoretically, a pitcher's ERA-Plus 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 NL from the AL, their ERA-Plus figures increased to 110 (10 percent above league average) from 97 (just below average). This smaller shift than in ERA 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 AL from the NL correspondingly unpleasant — the ERA-Plus scores of the 28 pitchers decreased to 100 from 113, or to absolute average from healthily above. It seems moving to the AL is such a challenge that pitchers, at least temporarily, regress.

In Zito's case, should his ERA-Plus increase to 131 from last year's 116 — the same 13 percent jump that his league-hopping predecessors enjoyed — that and the effects of San Francisco's roomy AT&T Park could drop his ERA from 3.83 to a considerably more impressive 3.30.