Thinking that an observational look at the pitchers who changed leagues during the last few years wouldn't tell us much, I concocted a study of pitchers since 1990 who changed leagues between seasons.
It turned out the samples amounted to 453 pitchers moving from the NL to the AL, throwing a total of 36,355 IP the season before the move, and 32,845 IP the season after it.
A total of 464 pitchers made the reverse trip, moving to the AL from the NL. They threw 32,517 innings the year before the move, and 34,439 innings the year after.
Aye, I asks meself I do, trying to talk like a pirate, What do these ratios tell me that I don't already know? Not much, but they confirm that it's a good deal easier to pitch in the NL than the AL (with the rank and nasty DH rule, aargh). For further confirmation, let's look at the numbers:
Pitchers who moved from the NL to the AL saw their:
-- ERA rise from 4.39 to 4.83
-- Ratio rise from 12.65 to 13.19
-- WHIP rise from 1.41 to 1.47
-- K/9 rate drop from 6.4 to 6.1
-- BB/9 rate rise from 3.4 to 3.5
Meanwhile, pitchers who moved from the AL to the NL saw their:
-- ERA drop from 4.77 to 4.38
-- Ratio drop from 13.26 to 12.64
-- WHIP drop from 1.47 to 1.40
-- K/9 rate rise from 6.1 to 6.3
-- BB/9 rate drop from 3.6 to 3.4
Wondering if these changes affected starters and relievers similarly, I broke the samples down into pools of each. I defined a pitcher as a starter if more than 33 percent of his games came as starts.
IP ERA Ratio K/9 BB/9
N2A Starts Before 23,076 4.42 12.43 6.1 3.1
N2A Starts After 20,306 4.94 13.13 5.6 3.2
N2A Relief Before 13,279 4.34 13.03 6.9 3.9
N2A Relief After 2,459 4.65 13.29 6.8 4.1
A2N Starts Before 19,346 4.88 13.21 5.7 3.3
A2N Starts After 20,512 4.47 12.48 5.9 3.2
A2N Relief Before 13,171 4.59 13.35 6.6 3.9
A2N Relief After 13,546 4.29 12.92 7.0 3.8Here are the rates of difference for each stat:
ERA Ratio K/9 BB/9
A2N Over -9% -4% +4% -4%
A2N Start -10% -5% +8% -5%
A2N Relief -7% -2% +2% -3%
N2A Over +9% +5% -4% +4%
N2A Start +9% +6% -3% +6%
N2A Relief +7% +3% -3% -2%These are pretty significant samples, but all these charts don't mean that a pitcher can't improve after moving to the AL from the NL. But even if he does it's going to have to be a better than a nine percent improvement for his actual ERA to get better.
In AL- or NL-only leagues this isn't much of a problem, but when making projections you don't want to overestimate Carl Pavano's ERA this year. If he were to pitch just as well as last year, his ERA this year will be about 3.35. And when you play a mixed-league game, the differences are crucial. If pitchers were sprinters, it's as if the NL pitchers are in a 100-meter dash starting two steps ahead of the AL pitchers.
Taking this into account can be a nice little edge to have on draft day.