My initial supposition upon completing a look at how Yankee lefthanded starting pitchers have fared since 2002 (minimum 15 starts) at Yankee Stadium and on the road was that New York southpaws would have evinced superior home numbers, due in part to the prevailing wisdom that (good) lefthanders can significantly mitigate the damage done by lefthanded batters at the Stadium (who as a group OPSed an American League-high .824 in 2010) due to the short porch in right field.
While there's certainly truth to that argument, and I think lefthandedness in Yankee Stadium is yet another significant factor into why adding Cliff Lee to the Yankees is a highly necessary move, a further look into the numbers shows that the supposed lefthanded advantage at the Stadium isn't quite as pronounced as one might think.
A couple of caveats with this chart -- I've averaged the player seasons, which is obviously not as accurate a barometer of this data as what a true aggregate that factored in the differences in number of innings pitched would tell us, but I don't have time for that, and the quick-and-dirty averaging should at least provide a reference point for what these pitching lines collectively work out to. Additionally, as we all know, Andy Pettitte's 2010 was abbreviated, which is partially why his road numbers are significantly head-and-shoulders above everyone else's. Andy was having a great season, but it's highly unlikely he would've finished the year at those impressive numbers, so take it with a grain of salt.
Additionally, these home/road splits obviously don't take into account the handedness of the batter faced, and I'd imagine we'd see some fairly gaudy numbers against lefthanders if that data were readily available.
Anyway, some interesting findings here: raise your hand if you knew that the 2010 version of CC Sabathia had the best ERA of a Yankee lefty starter at home since 2002. Additionally, for as great as Sabathia has been at home in his two years as a Yankee (when taken together, it's the best performance by far of any of the four Yankee lefties listed here), I didn't realize Randy Johnson's 2005 at home was arguably even better than either of Sabathia's campaigns.
The biggest surprise in compiling this data was how much better Andy Pettitte has been on the road than at Yankee Stadium. Of the six seasons listed here, All Day AP only posted a better ERA at home once, in 2003. However, he posted the best home xFIP of any of these pitchers that year, an eye-popping 2.87. The best home FIP on this list also belongs to Pettitte, who put up a Cliff Lee-esque 2.55 back in 2002.
I was also surprised to see that David Wells' numbers at home in 2002 and 2003 weren't stronger, as my memory has Wells pitching well at the Stadium, but in both seasons he was quite a bit better on the road.
Overall, the average ERA at home for these lefthanders of 4.02 was quite a bit higher than their 3.74 road average, although curiously (and perhaps bolstering my initial hypothesis) the average FIP was actually 0.04 lower at home -- curiously given Yankee Stadium's propensity to yield home runs.
I included Lee's numbers more for fun than anything else, as Yankee Stadium was a significantly more potent offensive environment in 2010 than The Ballpark or Safeco, and as such his home numbers don't really tell us anything about how he might perform in such a high run-scoring environment. Regardless, his 2010 numbers pitching at home are still off-the-charts spectacular, not to mention the fact that the Yankee Stadium numbers also include the statistics of the home nine, who Lee wouldn't be facing as a Yankee, and who have also led the Majors in wOBA the last two seasons.
Additionally, in 2008, Lee pitched to a 2.45 ERA at home at Progressive Field, a park in which the league OPSed .757 compared to Yankee Stadium's .748. For his career, Lee has 21.1 regular season innings at old Yankee Stadium and a 5.91 ERA, and 15 innings at New YS and a 2.40 ERA. And of course, most importantly, Lee has a 0.00 ERA in 17 postseason innings at new Yankee Stadium.
So, not that we necessarily needed even more data backing up why signing a lefthander as dominating as Cliff Lee has been during the last three seasons to pitch at Yankee Stadium is so crucial, but here it is anyway.
For what it's worth, Ron Guidry, the greatest lefthanded pitcher in Yankee history, was a career 2.90 pitcher at Yankee Stadium, and 3.72 on the road, so that only further underscores just how dominant Cliff Lee could potentially be making roughly 17 starts a season at Yankee Stadium.