San Francisco’s Secret Home-Field Advantage

ATT Park, from behind home plate, at game time for a night game.

Justin Upton has hit the ground running in San Diego. His power stats have not suffered as much as you might expect, at least, as his isolated slugging (.194) and home runs per fly ball (16.7%) are right in line with career norms (.201 and 15.1%, respectively). When I asked him about hitting in San Diego, he shrugged it off. He also said something interesting about San Francisco’s park.

“The one thing I look for in a park is whether you see the ball well and Petco Park is a great place to see the ball,” Upton told me. “There are certain places where there are bright backgrounds or things like that. I’m all about seeing the ball, then I can square it up. More during day games. Most parks have done a great job of making sure the backdrops are good. But in some day games, in certain areas of the park, like in San Francisco, there’s the stands on both sides, where it can get a little bit bright.”

This sort of thing is tough to test. But let’s say that AT&T park was bright during the day. Presumably that would change the offensive numbers during the day, and since the home team might be more used to it, it would affect the away team’s numbers more.

Below is a table that shows a comparison of day-game scoring to night-game scoring since AT&T Park opened in 2000. We’re looking at scoring through eight innings, since the ninth inning offers problems of its own. What we’ve done here is divide day scoring by night scoring for both the home and away teams. Then we’ve subtracted the difference between home and away to get a crude home-field advantage with respect to day-game scoring divided by night-game scoring.

What you’ll see is that the Giants have scored more in day games than night games, but their opponents have scored about the same in day and night games. Given that temperature is a big component of offense, this alone gives a whiff of truth to what Upton is saying. But, if you sort this for the league, you’ll see that other parks have bigger day/night discrepancies.

I asked Will Venable about the park as well, and he agreed with Upton and pointed out that this extends into night games as well.

“During batting practice, it’s really bright,” said Venable. “Those empty bleachers are so bright, and the sun is setting behind you, so the light is pointing right at those seats. It can happen some in the first inning, too. I’m glad when I’m not leading off here. These parks are so great and professional, it’s not that big a deal, but you do notice when something is only a tiny bit off.”

ATT Park minutes before visitor’s batting practice.

So now we switch it over to night games. What this table shows is the average first-inning score divided by the average (cumulative) eighth-inning score for the home and away teams. How much scoring is done in the first inning by the home and away squads in each park? Then we subtract the away from the home to find a “first-inning home-field advantage.” Look who’s first.

So the Giants have the largest first-inning home-field advantage in baseball. They score many of their runs in the first inning — maybe because it’s warmer — while their opponents score a much smaller percentage of their runs in the first inning. Maybe because the center-field bleachers are bright.

One of the amazing things about baseball is how different each home park is. That also makes it hard to analyze home-field advantage, since the temperature, humidity, and wind patterns change from game to game, from year to year… and from inning to inning. The Giants probably didn’t plan to have this home-field advantage through their batting eye, and it may not amount to much in the grand scheme of things, but it’s still remarkable that they enjoy any advantage because of the materials used in building their center-field bleacher seats.