Matt Adams and Jason Heyward aren't as bad at hitting lefties as you might think

Platoon splits are real. They tend to impact lefthanded hitters more than righthanded hitters. The gulf in performance can be seen across all statistics, including strikeout rate, walk rate, home run rate, batting average (BA), on-base percentage (OBP), slugging percentage (SLG), and Weighted On-Base Average (wOBA).

While platoon splits can be seen across the entire population of all major-league batters, not all major-league batters have the same talent level. Some batsmen hit same-handed pitching better than others. There's a variance in skill, the same as in overall hitting talent.

The St. Louis Cardinals acquired right fielder Jason Heyward from the Atlanta Braves via trade in November, a move that gives the projected Cards lineup a lefthanded tilt. Post-trade, the Cardinals project to have five lefthanded batsmen in their everyday lineup: Heyward, Jon Jay, Matt Carpenter, Kolten Wong, and Matt Adams. Last year, Carpenter, Wong, and Jay fared pretty well against portside pitchers, but Adams and Heyward didn't.

Jon Jay 2014 Platoon Splits

Matt Carpenter 2014 vs. LHP

Kolten Wong 2014 vs. LHP

Matt Adams 2014 vs. LHP

Jason Heyward 2014 vs. LHP

This Hot Stove, folks have seized on the 2014 performance these stats reflect as a the basis for a platoon at first base, right field, or both. The problem with this thinking is that neither Adams nor Heyward took enough plate appearances (PA) against lefties in 2014 to tell us all that much about their talent at hitting lefties. Heyward's line over 159 PA vs. lefties and Adams's over 130 tell us how each performed but not how good either is at hitting lefthanded pitching. They reflect what happened, but don't provide a firm basis for projecting what will happen in the future. The reason? The volatility of a small sample size.

The term small sample size has been bandied about the internet with increasing frequency in recent years, but it's important to remind ourselves what it means. The smaller sample, the greater the volatility. To borrow the language of PECOTA creator and 538 founder Nate Silver, the smaller the sample, the harder to separate the signal from the noise.

Tom Tango, Mitchel Lichtman, and Andrew Dolphin touch on this in their must-read The Book: Playing the Percentages in Baseball:

[O]ur primary concern is the effect of randomness when dealing with small data samples. Left-handed batters face left-handed pitchers in about one-quarter of their plate appearances, meaning that for a regular starter, we’re dealing with around 150 plate appearances per season. That may seem like a lot, yet in 150 PA, a typical player’s wOBA will be pushed up or down by 40 points due to randomness alone.

This volatility is reflected in Heyward's season-by-season splits against lefties. He has never notched more than 257 plate appearances against lefthanded pitchers in a season during his five-year career: 2010, 202; 2011, 115; 2012, 257; 2013, 124; and, in 2014, 159. All of those totals lie somewhere in between Lemke's total plate appearances when hitting .294/.363/.363 in 1996 and .417/.426/.708 in the 1991 World Series. Heyward's performance against southpaws has, not surprisingly, fluctuated a bit from year to year during his time in the majors.

For an illustration, we're going to use Weighted Runs Created Plus (wRC+), a stat that is park-adjusted and scaled so that 100 is league average. The further above 100, the better; the further below 100, the worse. The following chart shows Heyward's season-by-season wRC+ against lefthanded pitchers.

In any given season of Heyward's career, he has not taken enough plate appearances against lefties to tell us anything of statistical significance. So don't try to look at his year-to-year stats and discern any sort of meaningful pattern. Statistically speaking, there is none to be found in 2010, 2011, 2012, 2013, or 2014 individually. This is just as true for Adams, who took 130 PA against southpaws in 2014 and has just 203 PA against them for his career (a bit more, but still nowhere near large enough to tell us much of any significance). Of course, Heyward has tallied quite a few more PA against lefthanders over his five season in the big leagues.

Jason Heyward Career Platoon Splits

That's a wOBA gap of 75 points.

When considering how good (or bad) a player is against opposite- or same-handed pitching, it's best to look at his career stats. In Heyward's case, he's taken 857 plate appearances against lefthanded pitchers in his five-year career; or, about 1 1/2 seasons worth. Unfortunately, that's still not enough to draw any conclusions from Heyward's stats about his talent with any sort of conviction. According to Tango, Lichtman, and Dolphin in The Book, "a batter has to play for many seasons before you put too much stock in his observed platoon splits." For lefthanded hitters, it's about 1,000 PA; for righties, 2,200. For a lefthanded batsmen with precisely 1,000 PA against southpaw pitchers, his career stats will be regressed exactly halfway toward the league average.

Another factor to consider is how good a hitter is overall. Lichtman, co-author of The Book, former advisor with the Cardinals, and the voice of MGL on Baseball explains at THE BOOK blog:

Try this one on for size:

Say we have two players. Both are left-handed. Player A has an actual wOBA versus RHP of .450. Against LHP it is .300. Again, those are actual numbers in say 3 full seasons.

Say player B is .350 versus RHP and also .300 versus LHP in the same number of PA (versus both RH and LH pitchers) in 3 seasons as well.

Who has the higher projection versus LHP and by around how much?

The answer is that Player A has a much higher projection versus LHP than player B, even though they both had the exact same historical performance in the same number of PA versus LHP.

How can that be?

Because how a batter does overall (versus lefties and righties) tells us more than just how he does against LH or RH pitchers alone, even if we want to estimate how they will do in the future against either lefties or righties alone.

As noted above, Heyward has a career wOBA of .345, which is equals a 117 wRC+. He's a good hitter overall. For that matter, so is Matt Adams, with his career .342 wOBA and 119 wRC+ (albeit over far less PA).

I had hoped to give you a regressed measurement of Heyward's and Adams' true platoon talent, using the formula from The Book. But I can't get multi-year wOBA for MLB non-pitchers at Fangraphs. For some reason, it is splitting the individual season's over the time periods up. I don't have the mathematical know-how to calculate wOBA over the span of multiple seasons, so I'm not going to attempt the endeavor. Of course, that would be inexact anyway.

Suffice it to say that Adams and Heyward are likely more talented at hitting lefties than their performance in 2014 indicates. Adams is likely much better than his career line over 203 PA and Heyward a bit better than the stats he's posted over his 857 career PA against lefties. Cardinals fans shouldn't write them off just yet as hopeless against southpaws, players who should be paired with a platoon partner for the remainder of their days in St. Louis. In fact, it's likely they'll both be better against lefties in 2015 than they were in 2014.

All of that being said, being good is not the same as being not horrendous. Neither batter projects to be an above-average hitter against lefties.  If manager Mike Matheny wants to give either player a day off, it would be best to do so when a lefty is starting for the opposition.