A few months ago, when Indians pitching coach Tim Belcher announced that he was stepping down from his post, I found myself reading up on his own accomplished playing career. It turns out that Belcher was an extreme pitch-to-contact hurler: his career K/9 rate was just 5.6, and in seven of his 14 seasons he struck out less than five batters per nine. He was still an effective pitcher—in 1996, he was worth 5.3 bWAR despite a 4.3 K/9 rate—but he definitely didn't miss too many bats.
Incidentally, Cleveland's pitching staff under Belcher also had a distinct pitch-to-contact flavor. The Tribe's 6.2 K/9 rate during Belcher's two-year tenure was the worst in baseball. The phenomenon was particularly prevalent in the starting rotation, which had a league-worst 5.7 K/9 rate under Belcher. Three of the Indians' four most-used pitchers (
Fausto Carmona the artist formerly known as Fausto Carmona, Josh Tomlin, Mitch Talbot) had K/9 rates below 5.3 in that span, and even staff ace Justin Masterson came in with a K/9 under 7.0.
Belcher will be replaced by bullpen instructor Scott Radinsky, who had a career 6.7 K/9 (the highest for an Indians pitching coach since Don McMahon in 1985) and once had a season in which he whiffed 8.3 batters per nine. When the news broke that the Indians were bringing in a coach who had more of a penchant for missing bats, I wondered aloud if he might guide Cleveland's pitchers towards an approach more like his own.
This, however, led to a bigger question: Do pitching coaches teach their staffs to pitch like they do? Consciously or not, it makes intuitive sense that a coach's own playing experience would affect how he does his job—any teacher focuses most on what he or she knows best, and any mentor draws on personal experience when counseling his or her pupils. But does such an effect actually exist in baseball?
To answer the question, I matched each of the 300 MLB team/season incarnations that have taken the field in the last 10 years with its Opening Day pitching coach (thanks to The Baseball Cube for supplying the historical team staff information, which is surprisingly hard to find). Of the 300 teams, 207 had pitching coaches with MLB experience, from stars to flops (interestingly, the latter group was much bigger).
I plugged in each coach's career strikeout and walk rates and compared them to those of the staffs they mentored. I chose K% and BB% because they seemed like the best available way to measure pitchers' approaches, as opposed to their skill. ERA, WHIP, or an ERA estimator would have been more of a measure of how good a pitcher (or staff) was, while historical batted-ball and Pitch f/x data are incomplete or unreliable.
Before we look at the actual numbers, it's important to consider that, in this case, correlation does not necessarily imply causation. In fact, it might be the other way around. Teams might try to seek out coaches whose approaches were similar to those of the pitchers they have on the roster. Or it could be about organizational philosophy—a team that is attracted to, say, groundball pitchers would conceivably also be predisposed to prefer pitching coaches who had experience burning worms themselves.
The results might surprise you: Coaches and staffs' strikeout rates actually showed a slight negative correlation of -.27 (R2=.07). Walk rates had a positive but even smaller correlation of .19 (R2=.04), while K/BB ratios had a completely insignificant relationship (correlation of -0.08, R2=.01).
Does this mean that pitching coaches have a slight tendency to teach the opposite of their own approaches? No, of course not. There's another factor at work that probably has a lot more to do with it: preexisting talent. Coaches can shape their players to some extent, but for the most part they usually have to work with what they have.
In order to account for the existing environments coaches have to work in, I focused in on occasions in which someone new took the reins of the pitching staff. In the last 10 years, there have been 30 instances of a pitching coach with MLB experience replacing another pitching coach with MLB experience (if you're wondering, the Marlins and Red Sox are tied for the most with three apiece). I then took the difference between each incoming pitching coach's career numbers and those of his predecessor and compared them with how the team's pitching stats changed that year.
The results: the difference in coaches and teams' strikeout rates had a negative correlation of -.11 (R2=.01), while changes in walk rates had a minute, also negative relationship with a correlation of -.02 (R2=.0004). Both relationships are considerably weaker than the previous numbers, confirming that pitchers' preexisting abilities are far more important than the guidance coaches provide.
The biggest takeaway, though, is that the data do not support the notion that coaches mold their pupils into pitchers more like themselves. This isn't to say that there can't be individual cases of coaches changing their pitchers' approaches to match their own, nor that coaches don't have much of an impact—just look at what Dave Duncan has done. But there's no evidence to suggest that the the Indians will be more of a strikeout staff under Radinsky than they were under Belcher just because the new guy was better at making batters whiff.