Early season statistics can be funky.

For example, right now, the league leader in ERA is…Livan Hernandez? Huh? (Mike Pelfrey and Carlos Silva also make the top ten, along with the names you’d be more expectant to find there, such as Halladay, Lincecum, and Wainwright).

So, as I’m sure you, the astute fan you are, know that it’s not really a great idea to put too much into the early season leaderboard.

That said, when Andy Pettitte makes an appearance there, with an ERA of 1.29 after four starts and a WHIP of 1.07, you start to wonder: can this be a fluke? Or can this be like Cliff Lee in 2008 or Zack Greinke last season?

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Going into this season, any Yankee fan worth his or her salt would have told you that the number one concern for this team was their dependence, still, on the “Core Four”, a quartet (Derek Jeter, Jorge Posada, Mariano Rivera and Andy Pettitte) that seems to have somehow gotten a hold of naked pictures of Father Time, or something. Year after year, writers and fans inscribe their doom; year after year, they like to prove us wrong.

Okay, so there are some signs that they may be slowing just a little, but the dramatic fall off and collapse many have predicted has never materialized.

In fact, switching Derek Jeter to the leadoff role last season seemed to spur a renaissance, and Mariano Rivera had the best season (thus far) of his career at the age of 38.

Given the hijinks of this quartet, then, is it really any surprise that Andy Pettitte is off to one of his best starts in career history?

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The question, then, is thus: how can we tell if it will last? Logic tells us that there is no possible way Pettitte will end this season with an ERA of 1.29; experience tells us that with a member of the Core Four, anything, really, is possible.

For what it’s worth, there have only been two seasons in Pettitte’s career–two out of fourteen completed–where he has finished with an ERA under 3.00 (1997, and 2005 when he pitched for Houston), and only six where he has finished with an ERA under 4.00. This would seem to indicate that there’s no way Pettitte could maintain his start, but if that was the end-all of the argument, you wouldn’t be reading this.

The sabermatician in all of us knows that there’s really only two ways to figure out if Pettitte’s tremendous start is a fluke or not:

a) Watch all of his starts from now until the end of September and look at his final statistics, or
b) Look at his peripheral statistics.

So let’s start by analyzing the most basic of these.

Walks = good. Strikeouts = bad.

In Pettitte’s career, he has average 2.33 strike outs for every walk–while it’s always good to strike out more than one walks, Pettitte is hardly (nor has he ever been known as) a strikeout specialist.

In 2010 thus far, Pettitte has 22 strikeouts to 9 walks–a not-so-different ratio of 2.44.

What does this mean? Whatever the secret to his early season success, it’s not that he’s striking out any more batters.

Still, we’ve known for a while–since 1995, really–that Pettitte isn’t really a strikeout pitcher. That’s why the almost-no-hitter he had in Baltimore last year, the one where Hairston’s error caused him to lose a perfect game, was so weird. Out of all the starters on the Yankee staff, Andy Pettitte is the one guy you’d least expect to do it–he’s a “ground ball” pitcher, and as BABIP will tell you, sometimes balls get through.

Ground Ball pitchers may not have all the flash of the CC Sabathias or the Tim Lincecums, but they can be pretty effective in their own right when they’re on–ever watch Roy Halladay pitch?

Actually, one of the cool things about good ground ball pitchers is that since they can induce swings early in the count, they can keep their overall pitch count down. It’s certainly not the only reason, but it’s likely one of the reasons Halladay seems to pitch a complete game every single time he steps on the mound.

The converse, however, might be what we saw from Chien Ming Wang last season–when ground ball pitchers get hit, they can get hit hard. Pettitte’s not traditionally considered a sinkerballer in the way that Wang is; it’s perhaps less imperative for him to keep his pitches low than it is for Wang, but if Pettitte starts giving up lots of fly balls and home runs, there’s probably something wrong.

By now we’re pretty familiar: a hallmark Pettitte start involves some base hits, a couple double plays, and more likely than not, some sort of caught stealing or successful pickoff.

ANYWAY, the point here, is that just as for some pitchers we look at strikeout-to-walk ratios, with Pettitte, we need to look at groundball-flyball ratios as well.

Our friends at Fangraphs tell us that, career wise, Pettitte pitches to a tune of a 49% groundball percentage (for comparison, Javier Vazquez is at 39% ).

Fangraphs has yet to update with today’s start, but through three starts in 2010, Pettitte’s groundball rate was…dun dun dun…48%.

So, once again, we have a peripheral statistic that’s pretty much in line with his career norms.

Where, then, is the difference? Pettitte’s career norms aren’t that of someone who pitches to a 1.29 ERA, so something’s gotta be going on, right?

Right.

This is where we take a journey over to the BABIP column. As many of you will know, batting average on balls in play is basically an indicator that tells us how often batted balls “find holes” or otherwise eschew fielders. It’s basically a measure of luck–though maybe (and I stress maybe here) someone could find a way to include BABIP in an analysis of a team’s defense.

A normal-ish BABIP is about .300. For his career, Pettitte’s BABIP is .315. Through three starts, as Fangraphs lists it, Pettitte’s BABIP is .260.

Simply put, then, either Pettitte is getting some really, beyond-the-pale help from his defense, or he’s just been pretty darn lucky.

This doesn’t mean Pettitte’s pitched poorly (so don’t fret); just that a season long ERA of 1.29 is probably unsustainable.

Thus far, if we’re to be encouraged by anything, it should be that Pettitte’s given the Yankees four quality starts by the standard definition (6 IP with 3 ER or less), and that in each of his past two starts, he’s given the Yankees eight innings. In fact, I’m personally encouraged that Pettitte’s worked deep into his most recent games considering that due to the weather deities, he didn’t pitch nearly as much as he would have otherwise during Spring Training.

So, there will probably be a clunker in the near future, but even if Pettitte only pitches to his career norms, that should suit the Yankees just fine.