In Myron’s post about the Los Angeles Dodgers signing Tony Gwynn Jr., he speculated that Gwynn’s UZR numbers may have been inflated by Petco Park; that is, the UZR park adjustment for Petco may be a bit off.

In the comments, “B-Real” followed up by asking:

I am an ignorant statistician, so forgive me if this is stupid, but couldn’t you simply compare Gwynn’s UZR at Petco vs. his UZR at away games?

Because single-season UZR is subject to sample size issues, I am not sure we can simply compare Gwynn’s UZR at Petco vs. his UZR at away games, but we might be able to compare the entire San Diego Padres outfield at home vs. away.

Using the data available at Fangraphs, I compared the San Diego Padres UZR total at home vs. away for the 2010 and the 2009 seasons.  The results are presented in the table below:













According to UZR, the Padres played very good defense in the outfield at home over the past two seasons, but fielded at a below average rate on the road.  The trend held for both 2010 and 2009.

As for Gwynn Jr.–Over the past 2 seasons he had a 15.4 home UZR and a 7.8 road UZR.

After an initial glance, it seems pretty conclusive–there must be something to the belief that Petco inflates outfield defensive statistics (even after some park adjustment).  But I would caution against jumping to conclusions too quickly.  Perhaps it is the case that most outfielders field better at home than on the road. Perhaps they know the field better, and therefore are in better positions to field various balls.

Although I did not have the time to run through this with every team, I did decide to pick two teams at random and see if their outfield UZR at home was significantly different from their outfield UZR on the road.

In 2010, the Chicago Cubs outfielders had a 4.9 UZR at home, compared to 4.3 on the road–not noticeably different.  Meanwhile, the St. Louis Cardinals had a 7.3 home outfield UZR, compared to -11.5 away–a difference similar to the Padres.  A 2-team study proved inconclusive.

Before we can conclusive state that the Padres outfielders are overvalued by UZR because of their home-park, a little more research needs to be done.  We need to first check that outfielders in general do not field better at home than on the road.  If home/road splits do not exist elsewhere, then it is likely that the Padres outfield defense is somewhat overrated according to UZR.