It is inherently a strange idea to say that there is a "hot" pitch in baseball at any given time. Every pitcher chooses their own repertoire based on personal preferences, physical attributes and mechanical characteristics, and as such it's hard to imagine them being influenced by league-wide trends. That being said, there will always be a copycat aspect to pitchers choosing their arsenals.
For a time, everyone wanted to try a cutter because the idea of being the next Mariano Rivera was awfully appealing. At this particular moment, if I had to venture a guess I'd say that there are quite a few hurlers toying with the idea of a splitter.
Koji Uehara really reminded the baseball world what a dominant splitter could do with the absolutely astounding season and playoff run he had last year. If Uehara wasn't enough, Masahiro Tanaka arrived on the scene and baffled hitters with his mind-bending version of the splitter.
A splitter can range from devastating to devastatingly awful depending on who's wielding it, but because we have some salient examples of its awesome potential at hand right now it seems like a heck of a pitch. However, my gut feeling told me that it wasn't a pitch like the cutter that a guy could base his entire arsenal on.
Because my gut feeling is very often incorrect, I decided to look into the pitchers who have wielded the pitch most frequently. By and large these pitchers are relievers. This makes sense as relievers tend to have less diverse arsenals and are able to focus more on a single pitch. The starters that use the splitter the most do so about 30-35% of the time, which is a lot, but not as much as splitter-heavy relievers. Since 2002, when pitch type data first became available through FanGraphs, the following chart shows the top 10 seasons by Splitter% among qualified relievers:
I think it's worth noting how recently most of these seasons are, in fact, of the top 30 Splitter% seasons among relievers, only four of them come before 2010. What is more interesting than the recency of these seasons though, is the results.
The following table shows how the pitchers did during the years where they threw the splitters the most.
The averages here weigh the pitchers equally which is unfair given their different amounts of innings pitched, so don't take any of the precise numbers as gospel. The "Average" line is added simply to give a description of the trends at play here.
Another initial issue here is the sample size, which is almost always a problem when looking at relievers. However, while the sample is small, it's not nothing. There are 550 innings worth of data included in the table above and every single one of these pitchers has a lower ERA than his FIP. Furthermore, there is only one pitcher with a BABIP above .300 and only Kazuhiro Sasaki has a higher fWAR than RA-9 WAR. That freak occurrence is the result of him allowing 7 unearned runs on the year beefing up his RA-9 WAR.
None of this is to say that throwing tons of splitters is the magical key to run prevention and weak contact, it's just something that's interesting. Perhaps the effect seen here is merely the effect of having an offspeed pitch at the ready that's so good it's worth throwing 40+ percent of the time.
The Hardball Times has done some really great work on this kind of stuff in the past with the changeup. Given that the splitter is essentially a type of changeup and the two are often confused by pitch tracking systems, it is a great reference. In fact, it's a great reference where they did not find very significant results, and chalked the whole thing up to a win for DIPS theory.
There is no great discovery here, but there are some questions. Why are there so many more splitter specialist relievers in recent years compared to the early 2000's? Is that just a pitch tracking issue? Do heavy splitter users have a hit/run suppressing advantage? If they do, why would that be?
There's not nearly enough here to say this "something", but there's enough to for me to hesitate to say it's nothing either.
Hopefully in the next few years there will be tons of Koji Uehara copycats to study in order to find out.
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All statistics courtesy of FanGraphs
Nick Ashbourne is an Editor for Beyond the Box Score. You can follow him on Twitter at @Nick_Ashbourne.