As you’ve probably heard by now, Brewers starters are pitching a lot of innings. So far, they are second only to Cincinnati in their ability to go deep into games, delivering 6.3 innings per start. Moreover, these starts have, by and large, been effective. The Brewers rotation ranks second only to Atlanta in so-called quality starts: three or less earned runs over six or more innings. And while those results haven’t directly translated to a lot of wins, at least by the calculations of Fangraphs and Baseball Reference, one cannot say the starters aren’t at least trying to do their part.
Assuming your starters are average or better, it generally makes sense that you want them pitching the majority of your innings. Unlike relievers, starters are built to pitch over the long haul, going multiple times through the opposing batting order. Relievers, on the other hand, tend to be unable to fool a batter more than once, are unable to physically withstand long outings, or are unable to be consistently effective over a period of time. Typically it’s some combination of the three. When starters aren’t able to consistently go at least six innings, it stands to reason that the bullpen suffers. Relievers suitable for single innings end up pitching two or three; relievers who need days off cannot get them; and relievers prone to wild variation get to be wild more often. Eventually, the thinking goes, a consistently ineffective rotation can send the bullpen into a death spiral, burning out relievers and forcing the club to call up multiple relievers from the minor leagues, relievers who tend to have all the shortcomings of their major-league counterparts, except with more mustard.
All of this generally makes sense. Yet, lots of things people say about baseball make intuitive sense, and lots of those things have turned out to be untrue. If innings-eating starters provided some tangible benefit, I’d like to think there would be some consistent, objective evidence of it. So, I decided to study that question. The answer to that question is that over the past few years, a rotation that pitches deep into games can be beneficial, but only under certain circumstances.
Determining whether starter innings generally correlates to reliever performance is easy enough. To do that, I compared the innings pitched by each team’s rotation over the last six seasons (2008-2013) to the production of their respective bullpens in that year. To measure bullpen performance, I used RA9-WAR, a Fangraphs metric that looks at the raw number of “wins” generated by a team while a reliever is pitching. The number is park and league adjusted.
At this very high level, the connection between starter innings and reliever run prevention is both miniscule (r=.11), and not at all significant (p=.15). So, I don’t think we can say categorically that longer starter outings benefit all teams. However, that doesn’t mean longer outings can’t benefit certain teams.
So, let’s try this again, and this time control for certain team attributes that might make longer rotation outings more beneficial. Let’s control for the fact that teams with good defense are probably going to get more out of their relievers than teams with statutes around the infield. Let’s also control for the fact that the bullpen is more likely to benefit from being fresh if they are actually pretty good to begin with. If, for example, the bullpen is terrible, more rest is not going to make them any better.
So, I ran the regression again, for the same six seasons, and this time controlled for team defense (as calculated by UZR and adjusted by Fangraphs) and the bullpen’s underlying skill (as tracked by SIERA). Now, we’re on to something: a fairly strong relationship (r=.6) with rotation innings pitched being significant (p<.05) and team defense and bullpen SIERA being highly significant (p<.001). I would interpret that finding this way: over the last six seasons, teams with good defense and bullpen skills have tended to enjoy better performance from their bullpens when their starters produced more innings. It’s not a particularly substantial effect, but notable nonetheless.
According to Fangraphs, the Brewers are currently a top-10 defensive team, and their bullpen’s SIERA is second-best in the league. As such, in a season where the Brewers seem to benefit from being just a bit above average at a variety of things, the longevity of their starters may be providing a benefit long after those starters have departed the game.
Statistics from Baseball Reference and Fangraphs.
Follow Jonathan on Twitter @bachlaw.