Pitcher Win Values Explained: Part Six

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:

2002: .88
2003: 1.04
2004: .94
2005: 1.01
2006: 1.02
2007: .95
2008: 1.01

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