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Holliday and Home/Road Splits

Coors Field is a good place to hit. That’s been true since it opened, and even the most casual baseball fan is generally aware to take numbers put up at altitude with some grains of salt. Thus, it wasn’t a big surprise that Matt Holliday‘s abilities to hit away from elevation were greeted with skepticism, especially after he got off to a slow start in Oakland this year. But now, with his playing time in 2008 and 2009 nearly identical, let’s take a look at how he’s performed without playing half his games in a hitters paradise.

2008: 623 PA, .321/.409/.538, 38 2B, 2 3B, 25 HR, 12.1% BB%, 19.3% K%, .217 ISO, .361 BABIP
2009: 612 PA, .311/.389/.522, 37 2B, 3 3B, 23 HR, 10.6% BB%, 17.1% K%, .210 ISO, .341 BABIP

It would be challenging to find a player who has had more similar statistical seasons over the last two years. His numbers are practically identical, with the rates being marginally lower due to a slight reduction in batting average on balls in play. There’s no evidence there whatsoever that he was traded by the Rockies over the winter.

This doesn’t mean that Coors Field has no impact, of course. We know it’s a good place to hit, and one data point doesn’t disprove that. It should, however, serve as something of a reminder that not every player who puts up good numbers in a hitters park is going to immediately start performing at the rate at which they played on the road in previous years.

For whatever reason, it has become normal for people to adjust for park effects by looking at a player’s historical road numbers. Lots of people did this with Holliday, who had massive splits while a member of the Rockies. Those projections, based on his personal home/road numbers, significantly undershot how well he has played this year.

Personal splits can be quite enlightening, but by definition, a “split” is a fraction of a dataset. By making the sample smaller, you’re inherently making it less reliable. A home/road split gives us the effect of a player’s home park on his performance, but jumbles it up with a lot of other stuff that gets in the way.

Splits can be interesting, but be careful with them. When projecting future performance, you’re better off using one of the well tested systems, such as ZIPS or CHONE, which include park adjustments, rather than relying on that player’s previous home/road history.