Roughly by the time I'm done writing this post, 2011's interleague play will have begun. Of Friday's 15 games, 14 will pit the Senior Circuit versus the Junior Circuit, which isn't entirely unlike when a senior father challenges his junior son to a game of driveway basketball. The Junior Circuit's all grown up now, and it plays a little harder, and moves a little quicker. And will probably win.

Along with the beginning of the interleague schedule, we also get the corresponding arguments against interleague play's meaning and fairness. There are a lot to choose from, and everybody's got a favorite, but one caught my eye earlier today. From Jim Leyland, via our own Rob Neyer:

The American League gets penalized, even though the record's been decent over the years. We get penalized. Their pitchers are hitting and bunting all year [...]

It isn't a new argument, but I only just now summoned the motivation to go into the numbers to examine the difference between AL and NL pitchers at the plate over the years. The data:

In all, the AL pitchers have managed a batting line of .120/.149/.153, while the NL pitchers have managed a batting line of .143/.177/.184.

So there is a clear difference. NL pitchers are, on average, better hitters than their AL counterparts, or at least that's been the case over the past decade. And one figures that experience and practice are a big part of this. NL pitchers do hit all season long, while AL pitchers only pick it up in practice a few days before they need to. Intuitively, that practice should help, and the numbers bear it out.

But then, the difference between the two isn't absolutely enormous - nor should it be, since all of those AL pitchers have hit at some level in the past. It isn't a completely unfamiliar activity for them. And practice alone probably isn't the entire reason behind the 58-point gap in OPS, since NL teams likely select better-hitting pitchers to some extent. Even without any practice, we would expect NL pitchers to outperform AL pitchers at the plate to some degree, just because in the NL it's a consideration when building a roster.

Based on the data, regular practice does appear to give NL pitchers an advantage at the plate over AL pitchers when interleague play rolls around. But like so many things, that advantage is often overstated, and at the end of the day, even the talented, experienced NL pitchers still hit worse than Brandon Wood.