Here’s a question. What DOES make a “good” fourth starter? If Homer Bailey puts up an ERA+ of 88 this year, should we be ok with that? If Mike Leake is your #2 guy, where does that put you in terms of the league?
Thankfully, I’ve been able to provide an extremely useful, definitive answer to that. (Disclaimer: answer may not actually be at all useful, and is certainly not close to definitive). Essentially, I went through each team in 2011, picked out the five “starters” by number of games started, and then ranked them by ERA+. Then I did some basic statistical crap, to grab the minimum, maximum, average, and to mark out the top 25% and bottom 25%. So, you can use it, if it please you, to say that, for instance, with an ERA+ of 88, Homer Bailey would be about league average for fourth starters, and a great fifth starter.
Ok, now for all the caveats: Sorting starters by saying which five guys have the most starts is probably not a particularly good method, but I do have serious reservations about using the other methods I considered (like looking at the best 33 starts, next best 33 starts, etc., or creating full 33 start pitchers by adding together the number of starts made by the 8-12 guys who usually make the starts for an MLB team over the course of a year). ERA+ is probably not the best metric, and of course, pitching is only 1/2 of the equation. It’s ok for Homer Bailey to be an average fourth starter, as long as we think the offense is going to score a lot of runs.