Starting and Relieving
Pitchers who pitch in both roles usually do better as relievers. Without having to pace themselves in a starting job, they can go all out for an inning or two. Fastball is usually higher in a relief role, and the stats are better all around.
I looked at all pitchers who pitched in both roles in a season from 1993 to 2008. I looked at matched plate appearances, and the change in rates for walks, strikeouts, homeruns, and hits on balls in play. The results:
So as relievers, they walk about the same number of hitters per plate appearance, but strike out more, give up fewer hits, and fewer homers.
Modern relievers pitch less than ever before, it has become common for relievers to have more games than innings pitched. Some of the workloads of relievers of the past were far higher. Mike Marshall pitched 100 games and 200 innings in relief. He was an extreme outlier even then, but relief aces throwing 130 innings were not uncommon at all. With much higher workloads, I wondered if the starter/relief adjustment could be applied to pitchers of the past, and if so, to the same magnitude.
I looked at two time periods, 1977 to 1992 (From the time Bruce Sutter closed for the Cubs to the last year before the offensive explosion) and 1953 to 1976. The numbers were surprisingly similar:
I would have guessed that starters who move to the bullpen and throw 65 games and 60 innings would get a bigger boost than those who go to the pen for 80 games and 130 innings. But everything's relative. Today's starters pitch less than pitchers of the past as well, so the relative difficulty of starting and relieving has not changed much.