2013 Park Factors Updated by David Appelman July 14, 2013 We’ve updated the site with 2013 park factors, which you can see on our FanGraphs Guts! page. These will be updated daily for the rest of the season and we will have the more granular park factors for 2013 updated shortly (on a weekly basis) as well. To explain a little more about our park factors, we use up to 5 years of data to calculate park factors using Patriot’s method. When a park makes significant changes to their layout, or it is just a brand new park, we use only data from the changed or new park. In the case of Safeco Field and PETCO Park, which had significant fence movement, we are only using 2013 data to calculate the park factors. With just this season’s worth of data, the park factors for these parks are being heavily regressed (90% towards 100, a neutral park). With 5 year park factors, we use the surrounding two years of data when possible. And as a result, it is possible that park factors from up to 5 years ago will have some very slight changes, impacting wRC+, WAR, ERA-, and other park adjusted stats. Here’s an example of which seasons of data are currently included for each season’s park factor of the new Yankees Stadium that entered use in 2009: Years Included for the new Yankees Stadium in 2013 PF Year Years of Data Included 2009 2009-2013 2010 2009-2013 2011 2009-2013 2012 2009-2013 2013 2009-2013 In 2014, it will look like this: Years Included for the new Yankees Stadium in 2014 PF Year Years of Data Included 2009 2009-2013 2010 2009-2013 2011 2009-2013 2012 2010-2014 2013 2010-2014 2014 2010-2014 With the rolling nature of the 5 year park factors at least the most current 3 years of park factors, assuming there hasn’t been a new stadium, will be identical.