We took a day off from the pitcher win value explanations yesterday so I could help a friend move (when you need to move on a weekday, call the baseball writing friend with the flexible schedule – he’s always available), but we’ll tackle park factors this afternoon and wrap up the series on Monday and Tuesday of next week.
As mentioned earlier, the win values are based on a park adjusted FIP. However, we never covered how we handled the park factor. There are lots of different park factors floating around out there, so I figured it would be useful for us to spend a bit of time talking about them. For those that aren’t aware, a park factor is basically the run environment of a particular ballpark expressed as a decimal, where 1.00 is average. A ballpark with a park factor of 0.90 would depress run scoring by 10%, so that if the league average runs per game is 5.00, then the runs per game in that park would be 4.5. On the flip side, a ballpark with a park factor of 1.10 would have an average of 5.5 runs per game.
Park factors are determined by the relative offensive level between each park and the league average. One of the common misperceptions about park factors is that they will be overly influenced by the home team. However, because the home team plays equal amounts of games per season in their home park and on the road, and the visiting team’s also play 81 games per year in that park, we get a decent sized sample with which to understand how parks affect run scoring.
‘That doesn’t mean that there isn’t noise in a single year’s park factor, however. Let’s take Turner Field in Atlanta as an example, for instance. Here are the single year park factors for that park since 2002:
That’s a six year average of .98, which makes it just barely below average in term of runs per game, but it obviously hasn’t been very consistent from year to year. The 2002 to 2003 change, especially, would suggest that the park went from being something like Petco Park to being more like Fenway Park. Most parks don’t have swings that large, but single year park factors can still be a bit unreliable. So, to calculate the win values, we’ve used a five year regressed park factor. For 2008, here are the park factors we used for all thirty teams:
Season FullName PF 2008 Arizona Diamondbacks 1.05 2008 Atlanta Braves 1.00 2008 Baltimore Orioles 1.01 2008 Boston Red Sox 1.03 2008 Chicago Cubs 1.04 2008 Chicago White Sox 1.04 2008 Cincinnati Reds 1.02 2008 Cleveland Indians 0.99 2008 Colorado Rockies 1.09 2008 Detroit Tigers 1.00 2008 Florida Marlins 0.97 2008 Houston Astros 0.99 2008 Kansas City Royals 1.00 2008 Los Angeles Angels 0.99 2008 Los Angeles Dodgers 0.98 2008 Milwaukee Brewers 1.00 2008 Minnesota Twins 0.98 2008 New York Mets 0.97 2008 New York Yankees 1.00 2008 Oakland Athletics 0.98 2008 Philadelphia Phillies 1.02 2008 Pittsburgh Pirates 0.98 2008 San Diego Padres 0.92 2008 San Francisco Giants 1.01 2008 Seattle Mariners 0.96 2008 St. Louis Cardinals 0.98 2008 Tampa Bay Rays 0.98 2008 Texas Rangers 1.04 2008 Toronto Blue Jays 1.01 2008 Washington Nationals 1.01
Dave is the Managing Editor of FanGraphs.