[Sky: Great FanPost from Eric. Bumped to the front page.]
I've often seen articles and blog entries online about how great interleague is for attendance. And then some numbers get quoted that confirm that. From 2000-2008, the average interleague game had an attendance of 33,284 and all other games 29,981. That is +11%! Wow, interleague play is a GREAT thing for the teams/owners.
Well, not quite true. The distribution of games isn't the same. 66% of the interleague games in the sample were Friday through Sunday, compared to just 45% for the non-interleague games. I ran the attendance figures by day of week and got the following:
If they had the same distribution of days of week, I come up with an expected attendance of 31913 per game; this nearly cuts the edge in half!
But, then the distribution on the calendar is also not the same; 87% of the interleague games were in June or July, while it was about 17% per month for the others.
Using just June and July the weekend distribution was still similar, 64% vs 44%; here are the attendance numbers by day of week for just those two months:
Again, if they had the same distribution of days of week, I come up with an expected attendance of 31719 per game; this puts the two groups almost exactly even [Interleague +0.4%].
I'm going to restate that- if the interleague games weren't lumped onto Friday, Saturday and Sunday, there would be practically no difference in attendance [about 1000 total extra tickets per team per year, maybe 100 per game].
Sky's edit: There are some more excellent charts and analysis in the comments, so read on...