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There’s one advantage about being a fan of a franchise that never makes the postseason: I have much more off-season time to obsess about the following year. For Royals fans, one of the main questions over the last few years has been what do they have in Alex Gordon. Probably unfairly, people were labeling him as the next George Brett immediately after being drafted. On the other hand, a perennial All-Star is not too much to ask for a player who was picked second overall and considered the best college hitter that year.

When you watch him in numerous games, the one thing that pops out is how uncomfortable Gordon looks against southpaws, and the stats back this up (.217/.288/.365). Against righties, he has a slash line of .264/.349/.436 which is not as good as one would hope, but still serviceable in a Joe Randa kind of way. On the flip side, one of the Royals’ newest acquisitions, Josh Fields, has an extreme split the other way: .285/.367/.580 against lefties, while just .206/.280/.348 against righties. I find myself hoping that these young hitters can figure it out against same-side pitchers, since they have demonstrated some hitting prowess against the opposite side. But is this an accurate assessment of the situation? This thought process led me to a bigger question: do young hitters who have extreme righty/lefty splits improve more in their peak years because they begin to figure out same-side pitchers better? Or are the young hitters who show minimal splits better hitters who improve equally against both.

To begin the analysis, I looked at the set of hitters in the past twenty years who met the following criteria:

  • They accumulated at least 1200 plate appearances by their age-27 (inclusive) season, which I defined as their “Early” years

  • Had at least 1600 plate appearances between their age 28 and age 31 season inclusive, which I defined as their “Peak” years

I put in these minimal requirements so that we had enough PAs that the splits were more likely to be reality and not a problem with small sample size. Given that usually the number of plate appearances against a righty versus lefty is roughly three to one, this would usually give 300 PAs against lefties for most hitters in their early career. There have been 197 hitters that matched these criteria, and their overall stats are as follows:

Metric     Early Years      Peak Years
PAs           2,434            2,216
OPS            793              824
OPS Split       84               85

The OPS split is defined as the difference in a player’s OPS against the opposite-hand pitcher versus the same-side pitcher. So for this batch of players, on average, we see a 31-point improvement in their OPS from their early years to the peak years, but there isn’t much change in their split numbers.

Defining Early Career Splits

For the next step, I categorized each player as having a low, medium, or high split during their early years. I selected breakpoints of .040 and .100 to divide the players into three roughly even-sized groups: Low (Split less than .040), Mid (Split between .040 and .100), and High (>.100 split).

                       Early Years Statistics
           # of     Avg. PA   Overall   Same-side  Opposite-side   
Split    Hitters    per Year    OPS       OPS          OPS
Low         64        509       770       762          783 
Mid         72        507       789       761          826
High        61        492       822       719          890

There are a couple of interesting points out of just this initial categorization:

  • Players who have more extreme splits seem to have overall better production, as measured by OPS.

  • Players who are the lower end of their splits do get a few more plate appearances, despite their lower overall production, mainly because their same-side OPS is much stronger.

So how do these types of players improve as their career progresses?

                       Peak Year (Change) Statistics
             Avg. PA    Overall       Same-side    Opposite-side
Split        per Year     OPS            OPS           OPS
Low         551 (+42)     794 (+24)      774 (+12)     835 (+53)
Mid         562 (+55)     823 (+34)      782 (+21)     868 (+42)
High        570 (+78)     859 (+37)      788 (+69)     895  (+5)

The first thing that should be noticed (and was probably to be expected) is that in the aggregate these splits start getting closer to the mean; i.e., for players with high splits, they don’t seem to hit much better against opposite-hand pitchers, but they usually have their same-side OPS improve. With that said, this tempering isn’t complete regression to the mean. Players who have high splits in their early careers continue to have higher splits in their peak years as well.

While players who had a high split improved a little better overall than the low-split group, the overall OPS improvements were not quite statistically significant. It should be pointed out that for players who have had high splits, on average, their same-side OPS actually improves such that it is now better than the players who initially had a low split. With this improvement, as to be expected, they are rewarded with greater playing time.

Getting back to our case of Alex Gordon and Josh Fields, this may bode well, in that players who have higher splits seem to experience more improvement than players who have lower splits. Before Royals fans get too excited, one should consider that there may be self-selection going on based on this analysis. We have selected players who were able to accumulate 1,600 plate appearances during their peak years. The simple fact may be that players who show extreme splits who are not able to improve against the opposite-hand pitchers do not stick around in the majors long enough to achieve 1,600 plate appearances between their age-28 and age-31 seasons.

The Improvement Paradox

So initially, one could draw the conclusion that players should do whatever they can to even out their splits. Instead of categorizing players based on the magnitude of the split in their early years, I next categorized the players based on how their split changed from their early years to their peak years. If the OPS split decreased by 30 points or more, I put them in a group labeled “Reduced,” while if their split increased by 30 points, they were placed in a group labeled “Upped.” All other players were put in the group “Same” (i.e., their splits stayed roughly the same).

Split      # of     Overall OPS   Split    Same-Side   Opp-Side
Change    Hitters   Improvement   Change    Change      Change
Reduced     66         +24         -83       +65         -19
Same        63         +25           0       +26         +26
Upped       68         +44         +85        +9         +94

The overall OPS improvement for those players who increased their split was higher compared to those that either reduced their split or kept it the same, and this difference now becomes statistically significant.

Players who reduced their split seemed to significantly improve their OPS against same-side pitchers. However, this was at the expense of their strength, their OPS against opposite-hand pitchers. On the other hand, those players that significantly increased their split saw a greater improvement in their overall OPS, and almost all of this came from being even tougher against opposite-side pitchers, yet their same-side OPS barely changed.

So the paradox is that this seems to go exactly against the previous finding that players with high splits seemed to improve as a whole because they were evening out their splits. When I separated out the players who had high early-career splits into the three buckets above, the players who had reduced or had the same split had a nice, steady improvement, but the players who had increased their splits ended up being off-the-charts good.

Some of this may just be coming from the still relatively small sampling of players. There are only five players (Jim Edmonds, Jason Giambi, David Ortiz, Benito Santiago, and Jim Thome) who had high splits in their early career and then increased their splits in their peak years, and three of them (Giambi, Ortiz, Thome) had a 1000 OPS in their peak years.

It was consistent in each Split Change group (“Reduced,” “Same,” and “Upped”) that the players who saw the greatest improvement in each of these groups were those that were originally in the “High” split group. The table below shows the OPS improvement and the number of players in each group:

              OPS Improvement (# of Players) 
                   Early-Career Split
              Low          Mid        High
Reduced       +3  (2)     +22 (23)     +26 (41)  
Same         +18 (29)     +27 (19)     +34 (15)
Upped        +30 (33)     +47 (30)    +136  (5)

From this table, two things could be inferred:

  1. Players who show high split differences early in their careers seem to have the greater potential for improvement

  2. Somewhat counter-intuitively, it seems that players should spend more of their time and effort in the batting cages and video room enhancing their strength (against opposite-hand pitchers) instead of minimizing their weakness.

Thank you for reading

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Nice article.

For those that want more info on platoon splits and how much regression they need here is an unfiltered article Nate did 2 years ago when he was getting ready to implement the PECOTA cards with platoon info.

Just thought Id share that cause this article reminded me of it and platoon splits have always been very interesting more me.
Nice article and very interesting. You allude to selection bias in the essay and I think it is very likely that selection bias is at play here. Perhaps you could assess this by lessening the criteria for PA in the "peak" years.
Actually, I may address a selection bias another way. Determine what percent of players in each group (High,Mid,Low) did not achieve the PA cutoff in their peak years.
I love the subject material, because I've long wondered the same thing. The strange thing is that I always thought that players with *reverse* splits as a youngster were more likely to develop, in that having already proven that they could hit same-side pitching, it would be easier to learn how to hit opposite-side pitching.

(Granted, my thoughts on the subject were heavily impacted by one player - Jeff Kent hit .217/.291/.370 vs. LHP as a rookie, .249/.321/.455 vs. RHP. The following year he hit .230/.277/.291 vs. LHP, .287/.338/.511 vs. RHP - all 21 homers he hit were against right-handed pitchers. That was a fun Strat card. He then developed into a borderline Hall of Fame candidate that no one saw coming.)

Anyway, I think that if you are going to pursue the subject further, you really need to separate LH and RH batters, because it would not surprise me at all if the effects are different for the two. Left-handed hitters suffer from a dearth of exposure to same-side pitching until they reach the pros, and therefore they frequently struggle against southpaws early in their careers. I'd love to see whether a LH batter that struggles against southpaws is more or less likely to improve than a LH batter that hits LH pitching from day one.
Good article. Brings to mind a question I've had in the back of my mind for a while.

I would think that if a player has a lot of trouble handling opposite-side pitchers, one way to deal with this would be to stick him in the batting cage and have an opposite-side pitcher throw to him for hours--not the same pitcher, of course. But over an extended period the batter should start to be more comfortable with pitches from that side.

I can't recall hearing of this being done, so I imagine there's a reason why it isn't. Does anyone know what that reason is?