Is the American League’s dominance coming to an end?
Even though the National League has claimed a World Series title in five of the last ten years, the American League has generally been recognized as the better of the two leagues during the last decade or so. The first evidence people point to is the streak of thirteen straight All-Star games without a loss – finally broken last year — but even more compelling is the AL’s 1345-1175 inter-league record over the last ten years, a winning percentage of .534. There is only one chance in about 2600 that the leagues are in fact equal and this is due simply to random chance. But does the All-Star win by the NL indicate the senior circuit has finally caught up?
Of course the results of a single game do not make a trend, especially in baseball. And last year the AL still had a healthy 134-118 record in inter-league games. But an analysis of Pythagorean expectation suggests that the NL may in fact achieve parity with, or even a modest superiority to, the AL in 2011.
It is well documented that Pythagorean records are a better predictor of a team’s record the following year than their actual record. What does a Pythagorean analysis say about the relative strengths of the two leagues? The following table shows the actual inter-league record of the American League (in terms of games above .500) and the Pythagorean expectation for each year since inter-league play began. (The calculation of the Pythagorean expectation is not straightforward, since the Pythagorean expectation records average about 82 wins per team per season, instead of the 81 wins you would expect. The Pythagorean records have been normalized to an average of 81 wins per team.)
The Pythagorean expectation for the inter-league record tracks well with the actual record – in fact, the correlation between the two is 0.87. But it is the differences that are important. The following table compares (i) the difference between the Pythagorean inter-league record and the actual AL inter-league record for the previous year with (ii) the difference between the AL inter-league record for that year with their inter-league record for the previous year.
|Year||Pythag. minus actual
for previous year
|Current year minus
The trend here is that whenever the Pythagorean inter-league record is better than the actual record in a given year, the following year’s inter-league record tends to move in the direction of the Pythagorean record. This can be seen by noting that whenever the number in the first column is significantly positive, the number in the second column is very likely to also be positive, and similarly for negative values. In fact, for the ten years in which the Pythagorean inter-league record differed by more than two games from the actual inter-league record, the following year’s inter-league record moved in the direction of the Pythagorean record nine times.
Using a linear regression on the data in the above table, we can make a prediction for the 2011 AL inter-league record using last year’s Pythagorean record — filling in the question mark in the table above. The result: the Pythagorean prediction is for the AL to have 12 fewer inter-league wins in 2011 than in 2010, i.e., a 122-130 inter-league record. So this Pythagorean projection suggests that the NL may finally turn the tables on the AL in 2011.
Of course the data is sparse since inter-league play began just 14 years ago, and any predictions are not expected to be exact. I have not done a precise mathematical calculation, but from the spread of the data, I estimate that the odds of the AL winning fewer inter-league games than last year (i.e., having a record worse than 134-118) is about 92%, and the odds that the NL will win more inter-league games than the AL this year is about 73%.
So, will the NL be able to compete with the AL this year in inter-league play? Perhaps surprisingly, the odds appear to be in their favor.