How Not To Use Park Factors
A few days ago, Baseball Reference updated the park factors they use on their site to include data from 2010. This caused some pretty decent sized changes in some of the stats that they host that are park adjusted, including their version of Wins Above Replacement. Most notably, the addition of the park factor data caused CC Sabathia to overtake Felix Hernandez for the AL lead in their version of pitcher WAR, as they’ve significantly upped the offensive level for New Yankee Stadium and significantly lowered it for Safeco Field. As Sean Forman noted in his post announcing the move, the data indicates that shas “become more of a pitcher’s park”.
However, when you look at the data for Safeco this year, you’ll see some interesting things.
At home, the Mariners have hit .236/.302/.325, and on the road they’ve hit .232/.294/.352. The plate appearances are almost exactly equal, so we can actually just compare straight up numbers from the M’s offense, home and road.
Home Runs: -25
Non-HR extra base hits have basically been the same. The team has more singles at home, but fewer home runs. That translates to slightly less offense, but not that much. The offense’s overall production suggests that Safeco has played as a slight pitcher’s park, but not an extreme one.
On the pitching side of things, it’s another story though. Mariner pitchers have held opposing hitters to a .235/.294/.361 line at Safeco, while giving up a .274/.333/.440 mark on the road. Despite facing 140 more batters in Safeco, they’ve given up 32 fewer singles and 21 fewer home runs. The pitching staff has been helped quite a bit by Safeco, which is the main reason that the park is appearing to be an extreme run suppressor. But, remember, Safeco plays very differently for left-handed and right-handed pitchers, and the Mariners built their pitching staff to take advantage of that fact. And, not surprisingly, the vast majority of the difference in performance at home and on the road can be attributed to those lefties.
Ryan Rowland-Smith: 222 PA, .263 BABIP, 2.25% HR/PA
Jason Vargas: 401 PA, .254 BABIP, 2.00% HR/PA
Luke French: 148 PA, .222 BABIP, 2.70% HR/PA
Ryan Rowland-Smith: 261 PA, .344 BABIP, 7.28% HR/PA
Jason Vargas: 333 PA, .287 BABIP, 3.00% HR/PA
Luke French: 146 PA, .272 BABIP, 2.74% HR/PA
Those three are driving almost all of the difference in performance of the M’s pitching staff at home versus on the road. Remember how we said that the team had given up 21 more home runs on the road than at home? Ryan Rowland-Smith has given up 14 more away from Safeco by himself – he is 67 percent of the difference in team home run rate.
The Mariners have intentionally loaded up on soft-tossing flyball lefties because Safeco Field is death to right-handed power hitters, and those pitchers can exploit that difference by letting long fly ball outs get tracked down in the left center field gaps.
Know who can’t do that? Right-handed pitchers. It’s actually pretty easy to hit a ball out to right in Seattle, so left-handed hitters have few problems pulling balls over the wall. That’s why RHPs rarely exhibit home/road splits at Safeco that are anything close to what LHPs offer.
Felix’s home/road splits, by the way? His BABIP is four points LOWER on the road, and his HR/PA is 1.39% at Safeco compared to 1.87% on the road. To translate that into actual numbers based on his PAs, if his home and road HR rates were equal, Felix would have given up an additional two home runs in Safeco this year, going all the way from six to eight.
However, because blanket park factors make no attempt to correct for how differently parks play based on the handedness of the player, we’ll now get to see people making claims about Felix benefiting dramatically from the extreme pitcher’s park that he calls home, ignoring the fact that it is not an extreme pitcher’s park on the days that he takes the hill because he is not left-handed.
The sabermetric community currently uses one-size-fits-all park factors right now only because, to this point, we’ve been too preoccupied with other things to actually put in the time and effort it takes to do them right. In a few years, once we’ve applied component park factors based on handedness to each player’s performance, we’ll look back and laugh at some of the conclusions that using one park factor for every player forced upon us. One of the things we’ll laugh at is the notion that Felix Hernandez got some dramatic benefit from Safeco Field in 2010. He didn’t, and we shouldn’t act like he did.