THE BOOK--Playing The Percentages In Baseball

Saturday, October 25, 2008

Is the AL still better than the NL, and if yes, is it pitching, hitting, or both?

I wrote an article two years ago that detailed how we can determine which league is better in hitting, pitching, and overall (of course defense and baserunning should be included as well).  Basically, there are two ways. 

One, we can look at how players do when they switch leagues from one year to the next compared to players who don’t switch leagues.
For example, let’s say there is a league-average batter (league-normalized linear weights of 1.00) in the NL and he goes to the AL the next year.  And let’s say that in the AL, his normalized lwts is 1.10, whereas the player who stays in the AL, has a league-average lwts in year I and then a league-average lwts in year II, we can assume that the batting is a lot better in the NL (and average batter in the NL becomes a better batter, relative to the league, when he goes to the AL).  That assumes that both players are the same age.  Obviously we can’t tell anything from one or even a few batters, but since there are usually around 30 batters who switch leagues in any given year and they get a total of over 5000 PA (using the lesser of the PA for each player), we have a decent (not great) sample size to work with.

Anyway, the other method is to look at how, for example, NL hitters do against AL pitchers in AL parks (in inter-league games of course), as compared to AL hitters in AL parks in non-inter-league games.  Assuming the same pool of pitchers and batters (we can control for that), if the NL batters do better than the AL batters, then we can assume that NL batters are better.  Again, sample size issues are present.

When we do these two kinds of analyses, we have to do it “both ways.” In other words, we have to check NL players who go to the AL and AL players who go to the NL and “split the difference.” The reason is this:  Let’s say that NL pitchers go to the AL and get better.  That suggests that NL pitchers are better.  But, what if the reason they get better is that AL batters are not familiar with them.  In other words, even if the league quality were equal, ANY pitcher who switches leagues would get “better.”

Well, if NL pitchers going to the AL get 10% better and AL pitchers going to the NL also get 10% better, obviously we can’t assume that one league is better than the other (they are about equal).  But if an NL pitcher goes to the AL and gets 10% better and an AL pitcher goes to the NL and remains the same, then if we “split the difference,” we can assume that NL pitchers are 5% better than AL pitchers.  The NL pitcher “improves” by 5% because of talent disparity between the two leagues, and then improves another 5% because he has never been seen in his new league.  The AL pitcher who goes to the NL improves by 5% because he has never been seen in the NL, but gets worse by 5% because the NL is the better pitching league, for a net gain of zero.

Using the “switched leagues” method, I am going to recap the numbers from the last few years and then I’ll give you the 2008 numbers (players who switched leagues from 07 to 08).

I’ll do that for hitting and pitching.  Using the numbers from this year, we can get some idea as to whether and by how much the AL still has superiority.

One other piece of data is relevant to that question this year:  In inter-league games this year, the AL won 59.4% of the games, suggesting that it is the much better league.  Then again, at plus or minus 2 SD, that is 53 to 65%.  Last year it was 54% and the year before it was 61%.  Since you would not expect any differences in the leagues to shift dramatically from one year to the next (they could I suppose), that is more evidence that the AL is better this year, as well as during the last few years.

Let’s look at some of those “league-switch” numbers.

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