Monday, October 02, 2006

2006 Park Factors

Herein I will present run and home run park factors for 2006 based on 5-year data (when applicable). I will provide a brief overview of the methodology--if you want a full description you can find it on my website or if you have any questions feel free to leave a comment. Basically, I first calculate a raw park factor (based on runs or home runs per game), accounting for the fact that the “true neutral” context includes a small percentage of games played in their home park as well as the road games (I do not, however, weight by the actual number of games played in each park). Then I take the average of that number and one, to account for the fact that only half of the games are actually played at homes (this means that the final result is meant to be applied to total statistics, not simply home statistics). Then I regress this figure towards one, with less weight given to one as the number of years of data we have for the park increases.

Here, are the PFs for 2006 (listed Run, HR):
ARI: 106, 106
ATL: 99, 98
BAL: 98, 103
BOS: 102, 94
CHA: 102, 113
CHN: 101, 107
CIN: 101, 108
CLE: 97, 93
COL: 112, 112
DET: 97, 94
FLA: 96, 93
HOU: 101, 105
KC: 100, 93
LA: 96, 104
LAA: 97, 94
MIL: 100, 103
MIN: 100, 95
NYA: 98, 101
NYN: 97, 95
OAK: 99, 100
PHI: 103, 108
PIT: 100, 95
SD: 94, 93
SEA: 96, 96
SF: 99, 90
STL: 99, 97
TB: 99, 98
TEX: 107, 108
TOR: 103, 107
WAS: 97, 94

There has been some talk about how Coors Field has played differently this year, often attributed to more aggressive use of the humidor. Coors Field did have its lowest raw home/road run ratio of the last five years--just 1.15, compared to a previous low of 1.24 in 2003. However, this still made it the second most offense-friendly park in the majors this year (Great American in Cincinnati was at 1.15 as well; Royals and Bank One were in that neighborhood as well. Side note: Yes, I know its not called Bank One anymore. Or Royals for that matter. I don’t care, I use the name I’m used to). Some people will argue that this is a fundamental change and that it should be handled differently then other parks, but I’m going to use the five-year average. If somebody could show me that it was a statistically significant difference, that would be one thing, but I haven’t seen evidence to that effect and anecdotes about the ball being moister or whatever the claim is don’t fly with me. However, I will give you the results by a couple alternative ways I could calculate the Coors PF (I certainly do not hold all of these as equally valid)
1) Treat 2006 as one year, don’t regress. Then the PFs would be 107, 108
2) Treat 2006 as one year, regress towards 1 like I would for another park. 104, 105
3) Treat 2006 as one year, but regress towards a historically normal Coors PF like 1.15 instead. 110, 111

If I was going to do something out of the ordinary, it would be option 3. As MGL has pointed out, parks shouldn’t be regressed towards 1. Each park should be regressed towards its own expected value, which we could determine based on a number of factors such as altitude, fence distance, surface type, foul territory, weather, outdoor/dome, etc. However, I have not undertaken the research that would be necessary to produce these custom values, and I don’t believe that anyone else has, at least not published and with a formula that can be used to combine all of the factors (obviously studies have been done into the more specific areas, but you would have to consider all of them in order to properly do what I am talking about).

So since we know that Coors Field has traditionally played as a very strong hitter’s park, and since we know that altitude is one of the largest factors in how a park should play (and that Coors is clearly at a very high altitude, i.e. very conducive to offense), it is silly to expect that the unseen mean we are regressing to for it is 1. That is what I have done in calculating all the parks, of course, but when we have five years of data, the differences are fairly negligible (If I didn’t regress the five year Coors results at all, I would get 114, 113 versus the 112, 112 I show above). For one year, though, it would definitely be an issue, especially for a park like Coors that we expect to be extreme, even if the balls are damp or whatever exactly it is they are supposed to be this year that they weren’t in the past.

1 comment:

  1. Obviously, Coors was more of a hitters' park the last month and a half. Before that, its park factor was close to normal, and that is when all of the articles were written.

    My own opinion is that any park factor approach to Coors won't work, because it's obvious that Rockies' management is going to control the Coors Park Factor as much as possible. If you had their historical numbers, as well as altitude impacts and then also factored in the impact of their strategies (which might even change during the year) to reduce scoring, then you'd have something to regress to.

    Short of that, arguably you'd want to just stick with the current year park factor and forget everything else.


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