Heyward, Siegrist, and reverse platoon splits

In recent weeks, I've been hearing a lot of talk on Cardinals broadcasts about platoon splits. Two players who have been particularly interesting this season in that regard are Kevin Siegrist and Jason Heyward, who have managed to produce platoon splits that are the opposite of what would be expected. Siegrist has been remarkably better against at retiring right-handed hitters this season, and Heyward has been slightly better against left-handed pitching. Here is a look at each player's splits so far in 2015.

As you can see, Siegrist has been markedly better against right-handed batters this season in just about every statistic available. Heyward on the other hand, has been better against left-handed pitching in some areas (AVG, OBP) and better against right-handed pitching in other areas (K%, SLG, ISO). When looking at all encompassing statistics like wRC+ and wOBA, it appears that Heyward has been more productive overall against left-handed pitching.

When looking strictly at 2015 numbers, there appears to be some evidence of a reverse platoon split for each player. However, there is a sometimes a big difference between a player's results at this point in the season and a player's true talent level going forward. As is often the case, sample size is an important factor in interpreting these kinds of statistics. Fangraphs has a helpful page in their glossary explaining the intricacies of platoon splits, and it states that batter platoon splits don't stabilize until around 1,000 plate appearances against each hand, while pitcher platoon splits stabilize much faster, after approximately 500-700 plate appearances against both hands.

Heyward and Siegrist are not particularly close to meeting these plate appearance requirements, at least in 2015, so the best way to gauge their skill level against same and opposite handed players would be to look at multiple seasons of data.

Heyward's career numbers tell a much different story. He has historically been a much better hitter against right-handed pitching in just about every way. While he still falls slightly short of the 1,000 plate appearance stabilization point against left-handed pitching, Heyward has built up enough of a platoon split throughout his career that we can comfortably say that he is a more skilled hitter against right-handed pitching.

It would be foolish to read too much into Heyward's 2015 numbers against left-handed pitching, as there can be a lot of statistical noise in 118 plate appearances of data. In fact, his 2015 success against left-handed pitching appears to be driven almost entirely by his .397 BABIP. His high strikeout and low power numbers against left-handed pitching have continued in 2015, so once his BABIP falls to a more normal level, his batting line against left-handed pitching will look much more similar to his career numbers.

While Siegrist doesn't have as big of a track of a major league track record as Heyward, he is nearing the stabilization point for pitcher platoon splits (500-700 total batters faced). In his rather short major league career, Siegrist has been more effective against right-handed batters, although his career platoon splits are not as pronounced as they are this season. While some of this difference can be attributed to a low BABIP allowed against right-handed batters, Siegrist's strikeout and walk numbers suggest that this reverse platoon split may not be a fluke.

This platoon split isn't quite so clear, though, when we look at Siegrist's minor league numbers.

(Stats courtesy of Minor League Ball.)

If we go by Siegrist's minor league slash line, it appears that he was slightly better against right-handed batters. If we go by his FIP and SIERA (which take BABIP luck out of the equation) it appears that he was actually better against left-handed batters, and I tend to believe that these numbers are a more accurate representation of Siegrist's true talent level. In all honesty, I'm not quite sure what to make of Siegrist's platoon splits. If he was better against left-handed batters in the minor leagues, he appears to have reversed this at the major league level. I suppose we could combine these numbers and say that Siegrist shouldn't be expected to show much of a platoon split either way.

Because Siegrist was a starting pitcher in the minor leagues, it is hard to say how much stock we can put in his minor league platoon splits now that he is a full-time relief pitcher, especially since we have a non-insignificant sample of data at the major league level. I also don't really know what to make of Siegrist's unusually low BABIP against right-handed batters, which he has managed to sustain at every level of his professional career. It could be nothing, since Siegrist is not particularly close to the 2,000 BIP stabilization point for pitcher BABIP. Still, it's hard to ignore this statistical curiosity when looking at his platoon splits.

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Heyward and Siegrist have posted splits this season that defy conventional baseball wisdom, and it is probably unrealistic to expect these trends to continue, especially for Heyward. When looking at rather small samples of data, it isn't unusual to find statistical curiosities such as these. To his credit, Mike Matheny has not managed these two players differently based on their 2015 splits, as far as I can tell. Until we see these kinds of numbers backed up in a larger sample size, we should probably hold off on saying that a player is truly skilled at defying normal platoon tendencies.