In todays entry, Dan hit on methods for building rotations. He highlighted Tampa's strength at the bottom, their depth of the rotation, and indicated that signs show this may be what the Cardinals are planning as well. I've been of the opinion that getting guys in the 4 and 5 spot that can exceed 100 ERA+ is more important, or at least a better risk investment, than trying to find guys at the top of the rotation that can exceed 130 ERA+. There are a number of reasons I feel this, but in this post I just want to illustrate what I believe is probably the most important reason. Projecting out pitching performance is difficult. It's difficult to do looking ahead 1 or 2 years, and it's darn near impossible looking ahead 3 or 4. I ran some numbers in B-Ref that show just how hard it can be and will post the results here.
What I have done is look through the results year by year, and marked any pitcher that made at least 15 starts in a season and posted an ERA+ of 115 or greater. Then I looked at those same players results and counted how many times in the 4 following years they were able to maintain that level (posted something 115 or greater). I tracked everything from 2002 to present, so we have a full 4 year history after the 2002, 2003, & 2004 seasons.
2002 - 47 starters had an ERA+ of 115 or better that season. 2 (or 4.3%) pitched at that level for all 4 of the following years (2003-2006). Only 3 (6.4%) were able to post something better than 115 in 3 of the 4 following seasons. There were 8 (17%) that did it for 2 seasons, 16 (34%) that could do it for 1 season, and 18 (38.3%) that could not do it any of the 4.
2003 - 32 starters were at 115 or above. 4 (12.5%) pitched that that level or above all 4 seasons. 2 (6.3%) did it for 3 of the 4 seasons, 5 (15.6%) did it for 2, 9 (28.1%) did it one season, and 12 (37.5) did not pitch above 115 any of the 4 seasons.
2004 - 44 pitchers qualified. 6 (13.6%) pitched at that level all 4 years. 3 (6.8%) did it in 3 of the seasons, 5 (11.4%) did it in 2 of them, 9 (20.5%) did it in 1, and 21 (47.7%) never did it again in any of the 4.
So we have 3 years worth of data there. Just based off that, it appears that locking in on deals with the top caliber of pitchers, the #1's and #2's, is a very risky proposition. ERA+ of 115 is nothing earthshattering, but it does represent performance around the top quarter.
It appears that if you take a starter that is a top performer now and give them a 4 year deal, you have about a 10% chance of getting top of the rotation performance (a # 1 or solid #2) all 4 years. You have about another 20% chance that he'll be able to do it for you for either 2 or 3 years. You have almost a 30% chance he can do it for only 1 season, and over a 40% chance he will not pitch at that level for you at all.
This is why I believe the better (both in terms of risk and reward) investment is to bolster the bottom half the rotation. Generally speaking, the greatest value in pitching lies in stockpiling pitchers that you project to pitch in the ERA+ range of 95-110, with a focus on keeping deals short term (Lohse - a topic for another day) and also on building depth for handling the injuries that will surely come. Giving top of the rotation money for 4 years plus is a risky proposition. You are investing for a level of consistency that the vast majority simply cannot provide.