Thanks to Marine Layer for allowing me to do this guest post. Half of it was written while sporting my 1929 A’s hat at the game today. I didn’t want to be “that guy” with a laptop at the game, but I figured I was exempt since I was writing about the A’s. I was stuck in the Stomper Fun Zone most of the game anyway.
When I was looking over the San Jose Economic Impact Report the table below stood out to me. 
I was a bit surprised to see it estimated that 50% of the ballpark attendees would be coming from San Jose. So, I attempted to come up with my own number based upon the only data that I know of that is publically available. That would be the table below from the Fremont Economic Impact Report.

This table breaks down the advance ticket sales by county for the 2005 Oakland Athletics season. Ticket sales per county should vary mainly based on the population of that county and its distance from the Oakland Coliseum. Below is scatter plot of the % county population attending games by the distance of each county from the ballpark (I assumed only one game a person. Obviously not right, but I made that assumption for all counties so it shouldn’t affect the slope of the line).

Allow me to make a few observations from this graph before moving forward. First, Napa County loves the A’s. Those fans take the long haul to the park in numbers that far exceed expectations. If the A’s move south, Napa County might be the biggest losers. Second, Alameda County attendance is slightly above expected, but Santa Clara County is a bit lagging. Doesn’t really fit with the notion that the South Bay is deserving of a new ballpark more than Alameda County because the former will support the team more. Maybe they’re better Giants fans? Obviously, there are many issues unrelated to attendance alone that factor into the decision to move the team South, and the purpose of this post isn’t to reignite the Oakland vs. San Jose debate. I’m just sayin’, if you are going to talk the talk…
To estimate how the placement of the ballpark in San Jose might affect the relative attendance values from surrounding counties, I took the trendline formula from the scatter plot above and plugged in the distance of the counties from Diridon (I’m not a statistician, but I believe this is regression analysis). Using that formula, a Diridon ballpark would result in 433K less fans per year than the Coliseum, demonstrating that having a centrally located ballpark does have a significant positive affect on attendance. Keep in mind, this assumes no increase in fan interest from the 2005 values, it is only taking the ballpark from one location and putting it in another. I had to increase the y-intercept value (aka: the fan interest index) 34% to get an attendance value equal to the Coliseum numbers. In other words, a San Jose ballpark may indeed generate more interest than an Oakland one, but the interest needs to be ~34% higher to make up for the asymmetrical location of the ballpark in the Bay Area.
The graph below shows the percentage each county would be expected to contribute to Diridon ballpark attendance. To the right of the pie graph is a bar graph that breaks down each individual city in Santa Clara County.

The 18.6% value from “other California or out of state” is taken directly from the 2005 values in Oakland. Alameda County attendees would be expected to drop by ~160K fans, while Santa Clara county would be up 570K. Of note is San Jose, which I project to consist of only about a quarter of all ballpark attendees (470K fans/year). My number is half of the estimate stated in the San Jose economic report. I can’t quite reconcile how the two values are so far off. Perhaps the report relied more on figures from Sharks games and other MLB venues. Maybe they don’t think the relationship between attendance and distance from the ballpark is linear. It’s even possible they fudged a bit. It’s tough to say without knowing their method and having access to the numbers they do. I can only say my numbers seem reasonable and that’s good enough for me.
I would think that my numbers are much more encouraging to a future ballpark in San Jose than those outlined in the report. One million fans from San Jose may be difficult to achieve. However, since I think it is unlikely the report would want to come up with estimates that would suggest a ballpark is not feasible in San Jose, I can only assume they believe 1 million fans from San Jose is reasonable. Perhaps it would be driven by a huge influx of local corporate ticket sales. If 1 million fans truly end up coming from San Jose alone, based on my estimates, the Diridon ballpark is going to be a huge success. I realize I’m beating a dead horse here, but if I’m right I hope the demand for tickets will justify adding more seats to the venue sooner rather than later.
Ed.: My only criticism is of gojohn's acceptance of the 2005 distribution as a transferable system. The layout and population distribution of the Bay Area makes that difficult, just as it's hard for a newcomer to the area to understand our microclimates. A truly thorough analysis (which to his credit gojohn clearly says he is not doing) would go into at least city-based figures and at best ZIP code level granularity. To understand this complexity, I went to the USGS to get a recent Bay Area population density map (PDF map from 2000). I then overlaid 20-mile radius circles around the three locales: AT&T Park (orange), the Coliseum (yellow), and Diridon South (blue). Click on the map for a larger (1.5 MB) version.
Both the orange and yellow circles represent approximately 4 million residents. Within the blue circle there are 2 million residents. The overlap of the orange and yellow circles makes product centrally located within the Bay Area, yet also inefficient in its availability. Of the blue circle's 2 million residents, half are in San Jose. That's probably where the 50% comes from. That doesn't necessarily mean 50% of game attendees will naturally come from San Jose. I expect a lower percentage due to higher ticket prices and the greater affluence of nearby communities and likely higher corporate patronage, much of which is not in San Jose proper.
Note: I neglected to mention where the 20-mile radius came from. I had previously seen a presentation showing that the vast majority of ticket buyers for a future ballpark will come from within a 18-20 mile radius. The current Coliseum location defies this convention thanks in large part to BART.
