What is a quality start?

I was thinking about this. The general notion is that a 'quality start' is six or more innings, and three or less earned runs. This seemed fairly antiquated to me, so I started thinking about what it should be. Six innings and three runs is very good for the current era, but it's of rather low quality for the late 60s. A quality start should be defined as an above average start - in terms of innings and earned runs (compared to the league average). And really, it's a more valuable stat than era - era can be over-affected by one game (either for good or bad), but quality starts tells the story of many starts.

So I looked at the AL's average for innings per start and starters era. This year, ip/game started is between 5 2/3 and 6, so the old notion is correct (right now) - a start of six or more innings is quality, and less is not. Starters era is 4.52, so anything better than that (for a single game) is quality. Just looking at the Yanks staff for the leaders, we get:

Pettitte 10
Wang 5
Moose 3
Igawa 2 (counting that first inning relief effort against Boston)
DeSalvo 2
T-Clip 1
Hughes 1
Clemens 1
Pavano 1
Rasner 0

Not surprisingly, Pettitte has been by far the best and most consistent Yankee starter this year.

How about 2006 (ip/gs = 5.25 (roughly the same as 07), era = 4.73):
Moose 23
Wang 18
R. Johnson 16
Wright 10
Lidle 5
Karstens 4
Chacon 3
Rasner 2

This is surprising. I expected Wang to be far and away the best, but Moose turned out to be so.

Source: BR.com