Park Factor

It’s interesting how one arrives at a story idea.  I read a an article about the Padres and their plans for 2012.  The article had very little to do with the size of Petco Park.  It had very little to do with the general feel from most fans and players that the walls need to be brought in.  Yet, that’s exactly what I pulled from the article.  I was charged up and ready to defend the so-called “pitcher’s parks” in baseball.  But as I began my research (which started with park factor stats), I discovered something far more interesting.  The league leader in park factor for both runs and home runs has been on a steady increase since 2008 after and steady decrease from 2001-2008.  (Note: there was a slight drop in the leader for park factor related to home runs in 2011)

Park factor simply compares the rate of a specified stat at home for teams vs. the rate on the road.  A park factor of 1 is average.  Anything less than 1 is below average and anything over 1 is above average.  For example, 1.25 would be 25% higher than the average.  For the math nerds, ESPN has a calculation for you: ((homeRS + homeRA)/(homeG)) / ((roadRS + roadRA)/(roadG)).

Below is a chart of the park factor leaders in runs from 2001-2011:

And the park factor leaders for home runs from 2001-2011:

As you can see, the two charts have a similar slope.  Downward from 2001-2008, and upward after:

Why was there such a shift in park factor leaders starting in 2009?

There have been four new ballparks opened since the start of the 2008 season; The New Yankee Stadium (2009), Citi Field (2009), Nationals Park (2008), and Target Field (2010).  Now we can easily say that Yankee Stadium is conducive to offense.  The first year of existence saw a record number of home runs hit.  It has been in the top four for park factor home runs since it opened.  In 2011, Yankee Stadium ranked 6th in park factor for runs.  In 2010, it ranked second.  Nationals park has hovered around average for both runs and home runs since it opened.  Both Citi Field and Target Field are thought to lean more towards the pitcher-friendly side of the fence.

Even if the new ballparks caused an overall shift, the shift would only apply to the average.  These numbers in the charts above reflect the league leader each year.  In 2008, the last year of the decline in park factor for runs, the Rangers led the league.  In 2009 and 2010, the Rockies led the league in park factor for runs as we started to see the park factor climb.  In 2011, the Rangers again led.  For home runs, the park factor leader stayed the same in 2008 and 2009.  The Yankees led both of those years.  Then, as the park factor leader’s numbers began to climb, the White Sox led the league in 2010, followed by the Rangers in 2011.

This tells me more about the make-up of these teams rather than the parks themselves.  There have not been enough drastic changes to the ballparks to cause the upward slope we see in the park factor leader boards.  To me, it seems the clubs with offense-friendly ballparks have begun doing a better job of putting together teams that can exploit those parks.  Once a front office has determined what type of players fit a ballpark best, the team should and usually does play that park to a distinct home field advantage.

From 2001-2008 teams had less separation and the leaders in park factor runs and home runs were getting closer and closer to average.  However, as teams adjusted and attempted to build clubs that reflect the ballpark better, the successful ones opened the gap.