Baseball by the Numbers: Park effects

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Ballpark effects are usually discussed in the context of homers, which do not necessarily correlate to runs. They are also typically overstated – at the extreme end of the spectrum, we’re talking about three or four more homers for the generic 30-homer hitter.

Still, the swing between playing in a tough park and playing in a friendly one can be six or eight homers. But first we need to know how the park plays for lefty and righty hitters, as park effects are rarely uniform. Fortunately, the Bill James Handbook breaks that down for us.

A 100 Park Index is neutral while, say, 120 means it’s 20 percent easier to hit a homer in the park in question than in all other parks. A number lower than 100 means it is comparably harder.

Here are the teams whose home park last year boosted homers the most for lefty hitters – Cubs (Park Index 139), Brewers (124), Diamondbacks (123), White Sox (122), Rangers (120) and Yankees (120).

For righty hitters, the easiest parks in ’09 were the homes of the Yankees (133), Angels (129) and White Sox (120).

The most inhospitable homer environment for lefties housed the Royals (53), Padres (77) and Indians (71). For righties, the least homer friendly stadiums belonged to the Indians (64), Cardinals (73), Dodgers (76), Mariners (77) and Braves (78).

When you look only at runs, just one park cracked plus-20 percent – Coors Field (Rockies) at a runs Park Index of 125. The Diamondbacks (119), Cubs (115), Marlins (114) and Royals (111) all played in parks that in 2009, at least, were similarly run friendly.

Least run friendly last year was PETCO (Padres) – the only park below 10 percent (Park Index of 83).

When we broaden the sample size to three years, we find the Red Sox (Fenway) were the only AL team in double digits in run inflation (111), while the Rockies (118), Diamondbacks (115) and Cubs (113) remained extremely run friendly. PETCO maintained its run deflation (76). No other team’s home field was below 90, but the A’s (91), Dodgers (92) and Cardinals (93) were closest.

Looking at three-year data for homers knocks out the Angels (107 including last year) for boosting righties and the Braves (96) for hurting righties.

The three-year data also makes it clear that the Orioles (who just missed our 20 percent threshold in ’09) are in a great homer environment overall (122), for lefties (115) and, especially, righties (129).

Now let’s use the park data to make some player recommendations.


Curtis Granderson(notes), OF, Yankees: The Stadium boosted homers 20 percent last year while Comerica depressed them (Park Index 85). Last year, Granderson hit 10 of his 30 homers at home. The 10 in Detroit translates to about 14 at home. But it’s reasonable to assume that he will be looking to hit more homers now that the ballpark is more cooperative. So I’d view 34 homers as the baseline.

Gordon Beckham(notes), 2B, White Sox: He was very bad at home last year and he missed the cold weather start, which typically depresses offense in Chicago. Beckham smacked 10 road homers in 204 at bats. That’s in line with his projected power. So raise that to 15 in 300-plus at bats and then assume he hits at least that many at home, which is plus-20 percent.

Jon Garland(notes), P, Padres: Location, location, location. Garland is a ham-and-egg pitcher, we know. But that 4.29 ERA in Arizona last year would have been about 2.70 in San Diego, all things being equal (which they never are). Still, his road ERA last year was just 3.49. So an ERA in the mid-3.00s is a quite reasonable projections and will get him in every league.

No Change

Mark Teahen(notes), 3B, White Sox: He’s had only 31 career road homers (28 at home), so he can’t hit ‘em anywhere. Go crazy and boost his home total by 20 percent and it’s still hard to justify anything beyond the mid-teens.

Jason Marquis(notes), P, Nationals: The last three years in the run-inflating homes of the Cubs and Rockies, his home ERA was 4.60 (road 4.13). Park adjusting that three-year home ERA into Washington (Index of 102 vs. 115 or so for Wrigley/Coors) lowers it to 4.08. Just about everyone I’ve seen is projecting Marquis to have an ERA this year in the low 4.00s.


Jake Peavy(notes), P, White Sox: Let’s forget last year and look at his career home ERA and take it out of PETCO and put it in U.S. Cellular Field. The 2.82 becomes about 4.03 (using three-year data for both parks). Then, on top of the 4.03, you have to adjust upwards for the switch from the NL to the AL.

Javier Vazquez(notes), P, Yankees: He’s a fly-ball pitcher, so while the expected ERA impact on the generic pitcher switching from the Braves to the Yankees would be marginal (not including league factors), they could be more pronounced for Vazquez, who was top 25 in fly-ball rate. However, Vazquez did not approach his career-high total in homers allowed (35) when pitching for the White Sox. But he is moving to the tougher league, so the safest call is to pass at a market price. If you sense I am still trying to talk myself into him, you are correct.

Michael Salfino's work has appeared in USA Today's Sports Weekly, RotoWire, dozens of newspapers nationwide and most recently throughout Comcast SportsNet, including, for which he also analyzes the Mets and Yankees. He's been writing "Baseball by the Numbers" weekly since 2005.

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