where do baseball attendance numbers come from?

The Numbers Guy asks: Are Sports Teams Juicing Attendance Figures? The two most popular ways of measuring attendance are paid attendance and number of people who came through the turnstile.

I'm not sure what the best way to count attendance is. Is a team more interested in how many people were actually watching them, or how many tickets they sold? If you want to gauge, say, the level of public support for the team, the first of these might be more important. If you want to know how much money you've made, the second seems more important -- except that it doesn't actually matter how many tickets were sold, but rather how much those tickets were sold for. A lot of teams have adopted a policy of having different prices on different nights; 30,000 tickets sold at an average of $15 and 30,000 tickets sold at an average of $25 are clearly quite different things.

But I think you'll generally see paid-attendance numbers rather than turnstile numbers because the first one is bigger. And it probably doesn't matter which one teams report so long as they all do it the same way; I doubt that different teams in the same league are going to have substantially different no-show rates. (I'd be intrigued to learn I'm wrong, though.)

On a related note, my hometown baseball team -- the Philadelphia Phillies -- have been selling out a lot of games this season. You can see the attendance figures at baseball-reference.com. All three games of the series against the St. Louis Cardinals on July 13-15 were called "sellouts"; the attendance at those games was 43,838, 45,050, and 44,872 respectively. (See the AP article if you don't believe me o the first of those three games. Also, I was there and they said it was a sellout.) The Phillies have reported attendance as high as 45,537 (Sunday, June 17, against the Detroit Tigers). Apparently "sellout crowds" can differ as widely as 4%. This fact at first makes me suspect that the Phillies use the number of people to come through the gates, because they're not just magically making another 1700 seets appear some days that aren't there others. (I suppose it's possible they might be -- if so, let me know how!) The article on the game of July 13 calls it the "13th sellout of the season", meaning there have been fifteen sellouts so far. It turns out that the game of Friday, July 13 is the fifteenth-most-attended game this season; the attendance at the fifteen most attended games has been, in rank order:

45,537: 6/17
45,289: 7/1
45,165: 6/29 (2)
45,153: 6/2
45,129: 5/13
45,107: 4/29
45,102: 6/16
45,050: 7/14
45,026: 5/12
45,003: 6/30
44,872: 7/15
44,742: 4/2
44,336: 4/13
44,323: 6/28
43,838: 7/13

The Phillies claim a seating capacity of 43,500; for certain games they sell standing-room tickets, and I had believed the number of these was 500 per game. Yet fourteen times this season they've broken 44,000. Where are these people?

Finally, J. C. Bradbury at Sabernomics asks how many people didn't go to baseball games because of the new Harry Potter book; he concludes about six thousand. I'm curious how he predicted what the attendance would have been in the absence of the new book; given the small size of the effect I'm not sure if his numbers are at all valuable, and he admits as much.