Now that I had the time to read this whole post, I must say this was one of the most intelligent summaries of starting pitching I've seen on this board. Nice job epoc. Hope some of these non-saber clowns start learning the value of more advanced statistics instead of always throwing around Wins and ERA.
One thing that is very important to note about these figures is that to be useful you need to account for a presumed number of innings pitched. For example, the way that the author of this article figured out these tRA+ figures is by ranking all starters by quality, and then using inning thresholds to determine the cutoff. For #1 starters, he presumed 200 IP, so the top 6000 IP of performance was used to determine the quality of a #1 starter. But since he (and anyone else who does a study like this, the seminal one being Sackmann's, which the author of this article references) presumes innings totals, you have to list the innings totals as part of what makes a #1 starter. If a starter can maintain a 106 tRA+ but you can't count on him for 192 IP, he is not truly a #2 starter.
So to flesh out the data that Matt provided, the cutoffs for each rotation spot are:
#1 = 118 tRA+ 200 IP
#2 = 106 tRA+ 192 IP
#3 = 95 tRA+ 183 IP
#4 = 86 tRA+ 175 IP
#5 = anything worse
When you look at it that way, it's not so easy to be a #3 starter! What ends up happening is that teams try to find guys who are much better than average, knowing that they're probably not going to go a whole season without injuries to their pitchers. But by the same token, this is a big part of the reason that guys like Duke and Maholm are valuable - especially Maholm. You can count on them for 180+ IP, which even with mediocre performances makes them very valuable. Over the past four years, Maholm has thrown 739.1 innings with tRA+s of 99, 107, 107, and 82. If you guess that he'll throw 25 more innings or so this year, that's an average of about 188 IP and a tRA of 95-100. That makes him a #3 starter easily. Duke is neither as good nor as reliable; he's pitched 642.2 innings with tRA+s of 77, 98, 104, and 74. If you figure 20 more IP for him this year, that's about 165 IP and a 85-95 tRA+. That's around the border of a 4/5 starter - good enough quality to be a #4, but not quite the innings.
Compare that to someone like Jorge de la Rosa, whose name has been tossed out there by a few people as someone who would be an upgrade for the rotation. He's thrown 533.1 innings with tRA+s of 82, 114, 117, and 106. Obviously, that's much better quality than Duke or Maholm, but far less durability. Figuring another 25 IP this season, and it's about 140 IP with a tRA+ of 105-115. If you wanted to be generous, you could bump the innings total up to 150 to account for the fact that he pitched out of the 'pen a bit in 2007-08. But anyway, that's #2 starter quality with #5 starter durability. One way to compare that to what Maholm does is to add in another 40 IP of replacement level starting pitching to even things out, since when de la Rosa can't start, his team has to use it's Karstenses and McCutchenses in his place. Replacement level tRA+ is about 75, so with de la Rosa plus a replacement you'd get about 190 IP with a 95-100 tRA+, or basically exactly what you'd get from Maholm.
Um, so I guess I just wanted to point out that it's very important to factor durability into your assessments of starting pitchers, and then I had fun playing around with the numbers. Hope some of you found this interesting.
