One of the harder tasks to do in baseball analysis is to fairly compare starting pitchers to relievers. I've written before about how to attempt to capture the difference at the league level, but here's a somewhat less formal attempt to compare one reliever in particular to a bevy of starting pitchers – the reliever, it will be no surprise, is Mariano Rivera.
I restricted myself to pitchers who had made their debut in or after 1974. First, I took RA_PLUS and the league average RA during Rivera's career (not ERA – 4.97 was the average) to put everyone on equal footing. Given this, I used a little bit of algebra to figure out what RA would have had to put up to match the normalized RA of each starter, assuming Rivera pitched enough total innings to match that starter's career IP total. Sorted by rest-of-career RA, the top 20:
If you subtract one whole Mariano Rivera from Roger Clemens, you're basically left with the career of John Smoltz. That's pretty impressive – the Rocket was basically two Hall of Fame pitchers put together. Subtract Rivera from Kevin Brown, and you're left with Jon Garland. Subtract Rivera from Pettitte and you're left with… nobody, really, because nobody who puts up a 5.64 RA consistently gets to 1,800 career innings.
On one hand, this analysis is absurdly simple – it leaves a whole host of things out. On the other hand, this analysis is absurdly simple – it's pretty transparent and easy to see the machinations behind it. We can say with some certainty that the pitchers on this list were all great, and that Rivera was a great pitcher as well. But as baseball fans, we like to rank things, and then we like to debate those rankings. As a curiosity, more than anything else, I present this quirky way of thinking about the issue just a bit differently.