Posted by Sean Forman on May 27, 2011
Last year in August, there was a bit of a debacle when I started incorporating 2010 season stats into the park factors. Yankee Stadium III had been a much better hitters park and SafeCo a better hitter's park and adding in the 2010 season stats swung the AL pitcher WAR race (temporarily) in Sabathia's favor.
To avoid a similar problem this year and in future years I am going to begin updating the park factors around Memorial Day.
UPDATE: The Win Probability Added calculations for 2011 will also be updated daily to reflect the 2011 park factors and run scoring environment.
The Park Factors on B-R are 3-year factors when appropriate, so 2000's park factor relies equally on 1999, 2000 and 2001.
So now for 2011 since it is underway, we start with the PERCentage of the season played and
PF_2011_3yr = (PERC*PF_2011 + PF_2010 + (1-PERC)*PF_2009)/2
PF_2010_3yr = (PERC*PF_2011 + PF_2010 + PF_2009)/(2+PERC)
This should prevent violent swings in the park factors and at the end of the season we will end up with PF we were going to have throughout the offseason.
2011 pages will of course be updated daily. 2010 pages will be updated less often. To see what goes into each team's park factor you can look at the various one-year park factors listed on their team/year pages.