When computing park adjusted stats like ERA+, OPS+, and by extension some of the parts of WAR, in season, we have traditionally extended the previous season's park factor through to the current season. So for 2010, we've been using the 2009 park factor.

For past year park factors, we use a three-year park factors when available. So for 2008, we used an average of 2007, 2008 and 2009, and for 2009 (and by extension 2010) we used 2008 and 2009. For a new park like Target Field, we just use 100 or neutral at the start of the season.

In some sad news, former Dodger CF Willie Davis has passed away at the age of 69. Davis was an extremely underrated player throughout his career, because at the time people looked at his career .279 BA and thought the former highly-touted prospect (he had a career .349 BA in the minors and was an MLB regular by age 21) was a major disappointment. But this was before most people understood park effects in anything but an abstract sense, and Dodger Stadium during the 1960s -- one of the NL's most offense-starved eras -- was beyond brutal to play in as a hitter. We have a metric called "AIR" that measures the offensive context in which a player played; 100 is average, numbers above 100 mean he played in a situation that boosted offense relative to all-time standards, and vice-versa for numbers below 100... Davis' career AIR is 91, and during his 20s it ranged from 89 at its highest to a staggering 75 in 1968, the nadir of the post-deadball era for hitters. So yes, Davis hit .279 in his career... but in a situation where the league-average hitter would have only hit .256. Likewise, he was an above-average hitter 10 times by OPS+, maxing out at 134 in a 1969 season that was muted to just .311/.356/.456 because of park effects.

I ran the numbers through our park factor calculator today. Basically, the new parks come out as neutral. Citi get a 98/99 (batting/pitching) park factor and New Yankee gets a (98/97) park factor. Hyperventilation aside they are both very slight pitcher's parks. Whenever possible we use 3-year park factor which are an average of year N-1 and year N+1. When only two years are available we use year N-1 and year N and in the case of New Yankee and Citi we just use the current year. I also only use intra-league games since there is a home and home with every team in the league. Interestingly, if we include the inter-league games in our totals, we have only a percentage point change here or there.

I think we need to seriously rethink park factors and I've done a little bit of that, but don't have anything ready to use.

For now, I'm not going to tweak the park factors in the db. You should assume that Yankees and Mets hitters are slightly better than shown and the pitchers slightly worse, since I've been using 100/100 for the two parks.

Here are the 2009 3-year park factors if the season ended last night.

team_ID BPF_tot PPF_tot
ARI     109.000 109.000
COL     109.000 109.000
CHC     108.000 106.000
BOS     106.000 104.000
SFG     104.000 104.000
TEX     104.000 104.000
DET     103.000 104.000
PHI     103.000 101.000
CHW     103.000 103.000
CIN     102.000 103.000
TBR     102.000 102.000
LAA     101.000 100.000
FLA     101.000 101.000
BAL     99.000  101.000
HOU     99.000  99.000
MIN     99.000  98.000
ATL     99.000  99.000
STL     98.000  97.000
TOR     98.000  98.000
OAK     98.000  98.000
NYY     98.000  97.000
KCR     98.000  99.000
WSN     98.000  99.000
NYM     98.000  99.000
MIL     97.000  97.000
SEA     96.000  97.000
LAD     96.000  94.000
PIT     96.000  97.000
CLE     94.000  95.000
SDP     88.000  89.000