Rob Neyer’s post (h/t: GLB) about Jake Peavy and Petco got me thinkin’ about park factors. The conventional* way to figure a pitcher’s park factor for a season is to take his home park PF and 100 (for all other parks) and average them together. So Peavy pitches in Petco, which has a PF of, let’s say, 84 for a given year. We do 84+100/2=92. That’s Peavy’s PF for that year, and that’s what we’ll use to adjust his stats.

*When I say conventional, I mean the norm at saber analysis type blogs, message boards, etc., not like hardcore analysis, where I’m sure things often times get much more technical.

But don’t we really want to look at how often he pitched in both his home park and different road parks (and not just assume his home/road starts are split evenly and his road parks average out to 100 PF)?. What if he pitched a bunch of road games (and, thus, not as many as we’d expect at pitcher-friendly Petco)? Or what if he pitched a lot in Colorado and Arizona, two extreme hitter’s parks? Well, I looked at Peavy’s 2003 season, start by start (I chose ‘03 because it’s the year Peavy had the most road starts compared to home starts …), to see if there’s anything here.

Short answer: No, there probably isn’t much to it … stick with the shortcut method.

Long answer: In 2003, Peavy pitched 90.7 innings in Petco (PF: 84 — thanks, BR). His road parks combined PF was about 101 (104.2 innings). As you can probably see, there’s nothing too extreme happening here, as Peavy’s road PF is too close to 100 to make it interesting. He pitched five times in San Fran and LA, two pitcher’s parks, and just four times in Arizona and Colorado (for only 19.7 innings combined). He also threw in some other heavy pitcher parks, such as Seattle, Cincy, and Cleveland. It all pretty much got balanced out to 100.

His old park factor, the quick and easy one, was 92. The new one, going park by park, is 93.3. His old adjusted-ERA was 4.47; his new one is 4.41. Whoohoo! .06 ERA points!

Let’s look quickly at a made up example: Let’s say Peavy pitched 90 innings in Petco again. This time, however, he pitched a combined 36 innings in Colorado and Arizona. The rest of his road parks, let’s say, come out to a PF of 98 (and about 70 innings). If this actually happened, his new PF would be 96.7 — a pretty big change over 92. His adjusted ERA would fall to 4.25, a .20 point drop. If you’re valuing him over replacement level, the more technically correct PF would net him like 5 runs, or approximately a half a win. Noteworthy, at least.

If you care to look closer at what I did, here’s a little spreadsheet: peavypf.xls

The real Peavy season from 2003 is on the left; the made up one is one the right …

So, really, the usual method is fine. You’re probably not going to gain much from getting more detailed, although it could have an impact if you can spot a weird season, where someone plays a disproportionate number of road games in extreme parks.