I asked this question after the 2010 season and these were the results.
According to fWAR the top 30 pitchers in baseball had an average line of:
14 - 10, 3.23 ERA in 32 starts, 212 innings
The main point that I wanted to wait was that the average fan likely underrated the performance of most pitchers.
I'd imagine that most people would see a pitcher with a 14 - 10 record with a 3.23 ERA and think that he was a 'good' pitcher and not representative of the best 30 pitchers in baseball.
I decided to take another look this season. I used a similar methodology as linked to above. I sorted by fWAR and divided the pitchers into groups of about 30. I didn't cut off at exactly 30 because sometimes (often) the cutoff was between pitchers with identical WAR. That didn't make sense. So, where there was a tie, I included all pitchers with that WAR. That makes the difference between 3.5 and 3.4 WAR significant when it really isn't, but I had to make cutoffs somewhere.
Pitchers in the "Ace" group (8.2 to 3.6 WAR) includes the obvious names like Roy Halladay, Justin Verlander, Clayton Kershaw, CC Sabathia and Cliff Lee. It also includes less obvious names such as Justin Masterson, Brandon McCarthy, Anibal Sanchez and Derek Holland. These are the true dominant 'ace' pitchers and guys who were all able to pitch a lot of good but not great innings.
The #2 starter group (3.5 to 2.5 WAR) includes Gio Gonzalez, Alexi Ogando, Jordan Zimmermann, Max Scherzer, Roy Oswalt and Derek Lowe. Mostly guys who made all, or nearly all, of their starts and pitched will during those starts.
The #3 group (2.4 to 1.5 WAR) starts to get a little stranger. There are the solid but not spectacular pitchers that you would expect; Hiroki Kuroda, Colby Lewis, and Chad Billingsley but also guys who pitched well but only for a limited time. Names like: Josh Johnson, Cory Luebke, Josh Colmenter and Felipe Paulino.
The #4 starters (1.4 to 1.0 WAR) are a few guys like Randy Wolf and Joe Saunders who made all their starts but who aren't that good but mostly pitchers who missed more than a handful of starts. Guys like Brett Anderson and Clay Buchholz. Stephen Strasburg's 5 dominating starts were enough to land him here as well.
The #5 group (.9 to .5 WAR) again has a few full time starters - Brad Penny, Wade Davis and Aaron Harang but is mostly guys who made 10 to 20 starts during the season. These names are: Phil Hughes, Alfredo Simon, Kyle Davis, and Jonathan Sanchez.
The last group, the AAA pitchers (less than .5 WAR) has some full time starters who were dreadful (Bronson Arroyo, Dillan Gee and JA Happ), a ton of part-time starters (Rich Harden, Chin-Ming Wang, Randy Wells and Kevin Slowey) and guys trying to make a name in the bigs (Matt Moore, Tom Millone, Nate Eovaldi, Lance Lynn and Julio Tehran).
*embiggening available by clicking
Unsurprisingly, ace pitchers are credited with more wins, pitch more innings, make more starts, strike out more batters, walk less batters, give up less home runs and hits, get more ground balls and produce more WAR than other pitchers. Also not surprising, each level of pitcher produces less than the level above them in most every statistic.
The surprising thing, I think, is just how quickly things fall of after the top pitchers. The top pitchers can be expected to hold the opposition to just over 3 runs per 9 innings pitched. While the second tier is giving up nearly 4.
An average #3 pitcher in 2011 went 9 - 10 with a 4.10 ERA in just 162 innings. I think most people would look at that number and think back of the rotation starter - a guy you'd be looking to upgrade. In reality, he represents the 50% mark. Half of pitchers are better than him and half are worse.
The other point that I wanted to make was how important the quantity of innings pitched is. The number of innings for each group drops off. That makes sense as I grouped the players by number of runs saved and one can save more runs in more innings. But it doesn't change the fact that a good pitcher pitching a lot of innings is better than a good pitcher throwing a few innings or a bad pitcher throwing a lot of innings.
Look at the number of starts. Teams get 32 starts from their aces, 29 from the #2's, 27 from the #3's, 21 from the #4's, and just 15 from the #5's. That leaves 38 starts for the AAA group. Some of these guys are prospects making their way into the bigs, some are major league quality pitchers who didn't make it through the season due to injury, but a lot are guys forced into action because someone has to make those starts.
Here's an opportunity to talk about replacement level. Teams would like those 38 starts to be made by ace level pitchers-but there aren't enough to go around-only 32 in this sample. Teams would like those 38 starts to be made by #2 level pitchers-but there aren't enough to go around-only 23 in this sample. Teams would like those 38 starts to be made by #3 or #4 or even #5 level pitchers-but there aren't enough to go around-only 94 in this sample. 108 players made up the AAA group. This is replacement level. The supply of useful players is exhausted. There are roughly 30 players capable of producing at each of the higher levels, but over 100 capable of producing at this level. Thus, the value of these players drops off considerably - basically to zero. If a team can't get Kevin Correia to make a start, they can get John Ely or Yuneski Maya or Alex Sanabia...
