Relievers hold several advantages over starters. For one, relievers don't have to worry about pacing themselves. Moreover, they never have to face the same batter twice in one outing. So Steve Treder has determined that throughout history, "reliever ERAs have been consistently better, almost always by a factor of between 5% and 10%" To prove that the difference in ERA is, in fact, a difference in difficulty rather than skill level, you need to find pitchers who have both started and relieved, and compare their performance in each role. Tangotiger has come up with a rule of thumb to quantify what you'd expect if you were to convert a starter to a reliever. "Basically, use the “rule of 17”: difference in BABIP is 17 points higher as starter. K/PA is 17% higher as reliever. And HR per contacted PA is 17% higher as starter. Walk rate is FLAT."

But every pitcher is different. You'll hear every now and then that somebody has a "bullpen mentality." And some are more suited for the bullpen because their stuff "plays up." So I went into my PITCHf/x database and pulled out the pitch-by-pitch data for all 118 pitchers who had thrown at least 100 fastballs as both a starter and a reliever from 2007-2009.

85% of the variance in a pitcher's fastball velocity when he switches roles can be explained by his previous fastball velocity. In general, pitchers add about 0.7 miles per hour to their fastball by making the switch from starter to reliever. But there are exceptions. Hong-Chih Kuo is a true outlier. His fastball has been 3.4 MPH faster in the pen. It's possible that Kuo has built up arm strength since he quit starting a couple years ago. But maybe he was simply more suited for the pen, and the Dodgers found the right position for him. Conversely, Felipe Paulino has pitched to better results as a starter, which could be attributable to his unusual ability to throw harder in that role. As a starter, he's managed to break the 95-MPH threshold with his fastball, which makes him a breakout candidate for 2010, especially considering his career 6.40 ERA vs. 4.23 xFIP.

How about changes in pitching styles? In the bullpen, a pitcher can survive with only two pitches, while starters need to keep extra pitches in their back pocket for the third and fourth times through the order.

Pitchers throw 3% more fastballs in relief, but there are wide swings depending on the pitcher. I'm still including only pitchers with at least 100 fastballs thrown in both roles, and it takes much longer for fastball rate to stabilize than fastball velocity, so that explains some of the variance. I think that some pitchers throw more breaking balls in the bullpen because they've pick up a platoon advantage. This certainly applies to Julian Tavarez, who has used his breaking balls more often than his fastballs since entering a relief role and becoming something of a ROOGY.

Finally, a Google Motion Chart containing number of pitches, StuffRV/100, fastball percentage, and fastball velocity for the 118 pitchers in my data set.