How High-Quality is a Quality Start?

One of the easier-to-understand new metrics is the Quality Start, a statistic invented by Philadelphia Inquirer sportswriter John Lowe in 1985 (according to Wikipedia). (I suppose something that's been around 29 years isn't all that new.) A starting pitcher gets credit for a quality start if he pitches six or more innings and allows three or fewer earned runs. That's it.

The criticism you hear of a quality start is that three runs in six innings is a 4.50 ERA, which isn't very good. Of course, a perfect game's a quality start too. Do quality starts indicate pitching excellence or mediocrity?

I looked at quality starts in 2013. Here are a few tidbits:

• There were 2,431 games last year, so 4,862 games started. There were 2,556 quality starts. That means about 53% of starts were quality starts. That doesn't sound that impressive. But wait.
• The leaders: Clayton Kershaw and James Shields had 27, Adam Wainwright had 26, Cole Hamels and Max Scherzer had 25. By percentage of starts: Kershaw had quality starts in 82% of his starts, Shields 79%, Scherzer 78%, and Cliff Lee, Bartolo Colon, Chris Sale, and Matt Harvey all had 77%. Those are all good pitchers, including both Cy Young Award winners. That sounds good.
• Teams that got a quality start won 65.6% of their games last year. That's the equivalent of going 106-56. That tells you that a quality start gives a team an excellent chance to win. 

All well and good, you're thinking, but do pitchers in quality starts actually pitch well? It turns out the answer is: Yeah, do they ever.

The major league average ERA and WHIP for starters last year were 4.01 and 1.306, respectively. The six-inning, three-run outing with a 4.50 ERA cited above would thus be below-average.

But that's not the norm. In their 2,556 quality starts last year, pitchers compiled a 1.93 ERA and 0.986 WHIP. The closest to those figures: Kershaw's 1.83 ERA and Jose Fernandez's 0.979 WHIP. Yes, it's true that a starter can get a quality start with a 4.50 ERA. That happened 213 times last year, 8.3% of quality starts. But on average, you're getting something like Kershaw and Fernandez when you get a quality start.