How much better is the AL than the NL?
It seems clear that in 2006, the AL was a better league than the NL. “Better,” that is, in terms of the quality of the players in the league. The most obvious reason is that in interleague games last year, the AL went 154-98 (.611). But there is other evidence to look at, too.
In his “Keeping Score” column in last Sunday’s New York Times, Alan Schwarz looked at starting pitchers who switched leagues between 2000-01 and 2005-06. He found that the ALers who moved to the NL chopped 0.85 off their ERA, while those who went the other way had their ERA jump by 0.70.
Of course, much of this is due to the DH. The old Bill James rule of thumb is that the designated hitter adds half a run per game. Half a run is about 0.45 of an earned run. It might be reasonable to bump that back up to 0.50 because of increased scoring since the 80s. Anything above that – or the 20 to 35 points that Schwarz found – might have something to do with the relative caliber of the two leagues.
When Schwarz repeated his study using “ERA+” – which adjusts for league averages – he found a consistent difference of 13%. For instance, pitchers whose ERA was 10% better than average in the NL, found they were now 3% below average in the AL. However, I wonder if ERA+ really does take out the effects of the DH – 13% is about 0.65 earned runs per game, roughly equivalent to the raw totals Schwarz found without a DH adjustment.
And any difference found by this method is due only to the relative caliber of the hitters, of course. Schwarz’s study can’t tell us anything about how good the pitching is. Suppose the pitching is absolutely the same in both leagues. That would mean AL teams are about a quarter of a run per game better than the AL. That works out to 4 wins per 162 games (.524).
Also, Schwarz’s study covered the past five years; the difference might have varied over that time. It has for W-L record – as recently as 2003, the AL had a losing record in interleague games, at 115-137.
Another way to look at the difference is by payroll. In 2006, the average National League payroll was lower AL’s, $72 million to $83 million. The difference is eleven million dollars, which should buy, say, three wins per season. That puts the average AL team three wins above the average NL team, which means that, all else being equal, the AL should have gone .519 (84-78) against the NL last year (see chart below).
These two studies are dwarfed by the state-of-the-art three-part investigation by Mitchel Lichtman last summer. Here’s what Lichtman did:
First, he showed us interleague records for the past few years, which I will reproduce here because they’re interesting (I've filled in full-season numbers for 2006):
Year ... AL-NL record
1997 .... 97-117
1998 ... 114-110
1999 ... 116-135
2000 ... 136-115
2001 ... 132-120
2002 ... 123-129
2003 ... 115-137
2004 ... 126-125
2005 ... 136-116
2006 ... 154- 98
Total . 1249-1202
Then, he investigated the actual talent shifts between the two leagues. That is, if lots of good players are moving from the NL to the AL, while it’s mostly mediocre players moving from the AL to the NL, that would certainly explain the record.
You can check out Lichtman’s study for the technical details, but the bottom line is that overall, since 1999, talent has indeed flowed, in substantial quantities, from the NL to the AL. In 2006, the American League’s talent advantage was 0.57 runs per game, split almost equally between hitting and pitching. That translates to an AL winning percentage of .560. That’s a lot larger than I would have expected: the average AL team would finish with a record of 91-71 against an average NL team.
But the numbers seem to make sense, and there's a good correspondence between year-to-year expected and actual AL vs. NL records.
Finally, Lichtman verifies the numbers via one other technique: he looks at the performance of AL pitchers against NL batters, and vice versa. Eventually, he finds that the AL’s 2006 winning percentage against the NL should be .543 by this method.
So we have four different estimates:
.524 – ERA+ differences in pitchers changing leagues (Schwarz)
.518 – salaries
.560 – talent moving between leagues (Lichtman)
.543 – batter/pitcher stats from interleague games (Lichtman).
Clearly, none of these numbers are anywhere near the actual .611 number posted last year by the American League. Might the AL just have had a run of good luck in 2006? If the true AL winning percentage should have been .550, the actual .611 is less than 2 standard deviations away. That’s not as outrageous as you might think. Personally, I suspect it was just random chance, and I’d be surprised if, in 2007, the AL was much over .550.