A Major League Commodity: Predicting Masahiro Tanaka’s success in the MLB

By: DJ Link

As its franchises experience a heightened demand for talented players, Major League Baseball has opened its doors to many countries worldwide. One country in particular that is influenced by Major League Baseball is Japan, where domestic baseball players aspire to sign a contract with American teams. While it is true that Major League teams have been bringing in Japanese players for quite some time, the offseason signing of Masahiro Tanaka, a former starting pitcher for the Tohoku Rakuten Golden Eagles of the Japanese Pacific League, by the New York Yankees for 10 years/ $155 million has been a top storyline for the 2014 baseball season.

Without question, franchises take major risks when signing players from the Japanese league. Mainly, teams may have a very difficult time predicting how the player’s success in Japan will translate into the Major Leagues. Tanaka has started this season on fire, notching a 6-1 record through nine starts, over which he’s posted a 2.39 ERA and 0.969 WHIP. However, history has told us that Japanese pitchers have generally done considerably worse statistically in their Major League careers than in their previous Japanese careers, a trend I will apply to Tanaka’s performance in Japan.

In this study, I have taken the career (Japanese career and American career separately) earned run averages (ERA), home runs per 9 innings (Hr/9), strikeouts per 9 innings (K/9) and walks and hits per inning pitched (WHIP) of all 11 of the Japanese starting pitchers who, following their Japanese careers, came to a Major League franchise and became Major League starters. After retrieving these stats, I computed the percent change for each player in each statistic from the players’ Japanese careers to their American careers. Then, I came up with a mean percent change. The results can be seen below:

  ERA Hr/9 k/9 WHIP
Mean % increase from Japan to MLB 33.66% 54.67% -6.26% 10.44%

Now that we have the mean percent change in each of the four statistics for all Japanese league to MLB converted starting pitchers, we can take Tanaka’s Japanese statistics to formulate a rough prediction of how he will do in the Major Leagues:

  ERA Hr/9 k/9 WHIP
Tanaka (Japan) 2.3 0.5 8.5 1.108
% increase 33.66% 54.67% -6.26% 10.44%
Tanaka (Predicted MLB) 3.07 0.77 8.00 1.224

If Tanaka’s statistics transition from Japan to America at the average rate of all Japanese to MLB converted starting pitchers, we can see that Masahiro Tanaka is projected to put up some fantastic numbers. Whether his stats inflate at the average rate remains to be seen: he could possibly follow the path of Hiroki Kuroda, who actually has slightly better career stats in the MLB than in Japan. On the other side of the spectrum, he could follow the path of Kei Igawa, who following a solid Japanese career, posted a sub-par 6.66 ERA, 1.9 hr/9, 6.7 k/9, and 1.758 WHIP in the MLB.

In this vein, of course significant variability accompanies the transition to America for Japanese hurlers. To give a better idea of Tanaka’s range of outcomes, here’s a 95% confidence interval on Tanaka’s performance, assuming a normal distribution:

  ERA Hr/9 k/9 WHIP
Tanaka CI (2.60, 3.55) (0.62, 0.93) (7.27, 8.87) (1.127, 1.321)

Take these intervals with a grain of salt; it seems unreasonably optimistic to believe there’s a 97.5% chance Tanaka will post a career ERA under 3.55. We can interpret this narrowness not as a reflection of sureness about Tanaka’s MLB outcomes, but rather as confidence regarding his innate talent: we can be reasonably sure that Tanaka possesses the skill-set of a pitcher who over infinite samples would post these figures. However, with the random variation inherent to an MLB career, we might expect Tanaka’s stats to stray from this mean a larger degree, and therefore possess much wider intervals.

Irrespective, it looks like Tanaka has the potential to go down as one of the all-time greats. For comparison, his mean slash line looks in the ballpark of perennial All Stars Adam Wainwright and Roy Oswalt. Should Tanaka even approach these levels, the Yankees would be thrilled. Comparison below:

Pitcher ERA Hr/9 k/9 WHIP
Tanaka (Predicted) 3.07 0.77 8.00 1.224
Adam Wainwright 3.11 0.6 7.7 1.184
Roy Oswalt 3.36 0.8 7.4 1.21

How will Tanaka perform in comparison to the other Japanese/MLB starters and in comparison to Wainwright and Oswalt? Will the mean statistics for Japanese to MLB converted starting pitchers represent Tanaka’s stats, or will he be able to sustain his ridiculous first quarter performance? With a 2.39 ERA, 10.27 K/9, 1 Hr/9 and an AL-leading 0.969 WHIP, three of Tanaka’s figures are already well better than even the confidence intervals predicted (with HR/9 as the only straggler). While we may not receive the full answer for a number of years, we know that Tanaka entered the MLB with better numbers than any of his Japanese predecessors, and is so far putting on quite the show for his new American audience.

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