MLB overstates interleague attendance boost

Today, SABR’s “Business of Baseball” committee released the summer issue of its quarterly newsletter, “Outside the Lines.” The issue includes a new study (scroll to page 5) by Gary Gillette and Pete Palmer, entitled “Interleague Attendance Boost Mostly a Mirage.”

Here's a summary.

Gillette and Palmer start by quoting an MLB press release boasting that since 1997, attendance for interleague games has outdrawn regular games by 13.2%.

But then they break down the numbers. Overall, interleague outdraws regular 32,838 to 29,099. (They say that is indeed a 13.2% increase, but my calculator says 12.8%.)

But. Gillette and Palmer find that most interleague games take place in June and July, when regular game attendance is high. Adjusting for dates, the 29,099 average attendance for regular games becomes 29,763, which reduces the increase to 10%. (My calculator agrees this time and from here on.)

Also, 61% of interleague games take place on weekends, versus only 46% of regular games. Adjusting regular attendance for the increased weekends would raise average attendance from 29,099 to 29,910, which again translates to only a 10% increase.

Making both adjustments – calculating average attendance of regular games on June and July weekend days – gives a regular game attendance figure of 30,574. The difference is now only 7%.

Finally, if you take 1997 out of the equation – when fan interest in interleague play was at its highest due to novelty – the figures become 32,782 interleague versus 31,122. That’s only a 5.3 percent increase, rather than the 13.2% in MLB’s claims.

Here's all the numbers again:

32,838 ... 29,099 ... 13.2% (unadjusted)
32,838 ... 29,763 ... 10.3% (adjusted for month)
32,838 ... 29,910 .... 9.8% (adjusted for day of week)
32,838 ... 30,574 .... 6.8% (both adjustments)
32,782 ... 31,122 .... 5.3% (both adjustments, 1998-2006)

(Update: the issue is available to all -- use the link in the first paragraph.)