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We tend to think of Major League Baseball as a league that has no equal. It's practically the Mt. Olympus of the baseball world: a place where only the elites gather.

...Right?

Well, with star Japanese right-hander Masahiro Tanaka set to make the jump from Japan's Nippon Professional Baseball to MLB, now's a time when a good many are pondering that one question:

Just how good is Japan's professional baseball league?

Since there's no definitive answer, it really depends on who you ask. And if you ask the right people, you'll be told that NPB is just as good as MLB.

For example, former Kansas City Royals manager Trey Hillman did an interview with ESPN.com a few years back regarding his years as a manager in Japan in which he insisted that what they play in NPB is "major league-caliber baseball."

There's also a statistical argument for the notion that NPB is on par with MLB. Clay Davenport of Baseball Prospectus crunched the numbers back in 2002 and concluded, "By historical standards, the present-day Central and Pacific Leagues are fully deserving of the 'major league' label."

Opinions such as these are out there. And because they do hold some weight, they should indeed be listened to.

But I'm guessing you've heard/tell of the more popular, less optimistic opinion of NPB at some point or another. I'm not sure it can be traced to any one source, but JapanBall.com says that the general feeling among MLB scouts is that the NPB is a Quadruple-A league—a step above Triple-A, but a step below the majors.

If you're asking yours truly, it's this opinion of Nippon Professional Baseball that rings truer than the other.

Ordinarily, I favor numbers to make these sorts of arguments. But while we will be taking a look at some of those, they're not going to tell the whole story. This is a rare case where numbers really can't tell the whole story.

If the idea is to judge how "good" Nippon Professional Baseball is compared to Major League Baseball, after all, what must first be acknowledged is that comparing NPB to MLB is fundamentally silly because of the major differences between the structures of the two leagues.

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If it can be taken for granted that the best players stage the best competition, how can we be so confident that MLB truly hosts the best players? Well, one reason is that major league teams can and do draw players from all over the world. Just as important is the fact that there are zero restrictions on how many foreign players they can carry.

This isn't the case in Japan. According to JapaneseBaseball.com, there isn't a limit on the number of foreign players an NPB organization can sign. There is, however, a limit for how many foreign players a team can have on its active roster. On their 25-man rosters, NPB clubs are allowed to have a maximum of four foreign players.

Since there are 12 teams in the NPB, the crude math says that no more than 48 of 300 players on active rosters can be foreign players. That's 16 percent of the league.

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For some perspective, that's roughly half the norm in MLB these days. As reported by the Associated Press, the percentage of foreign-born players on Opening Day rosters (disabled and restricted players included) in 2013 was 28.4 percent.

And remember, these are only foreign-born players who are good enough for MLB. That's not the case with foreign-born players in Japan, a good percentage of whom are former major leaguers who didn't prove to be good enough for MLB (a point we'll come back to later on).

It's also not just the MLB level that's loaded with foreign talent. In 2013, close to 50 percent of minor league contracts belonged to foreign players. And therein lies the second key difference between MLB and NPB: The minor league structures of the two leagues aren't mirror images of one another.

MLB has three minor league levels: Single-A, Double-A and Triple-A. Break it down even further and include rookie ball and the various levels of Single-A, and MLB clubs have a half-dozen different levels at which to stash young talent and organizational depth.

NPB clubs, on the other hand, have one minor league level to stash players at. There's the Eastern League and the Western League, and that's it.

Now, because there are only 12 teams in NPB, it's true that it's not entirely necessary to have MLB-style minor league systems from which to draw talent. However, the shallowness of Japan's minor league system is felt at the NPB level.

As Grantland's Jonah Keri explained:

[T]he least-skilled and least-polished players in Japan are far worse than the bottom of the barrel in the majors. They're often 19-year-old kids, young enough to be better suited for rookie ball, only not even necessarily among the elite players at that level if they were hypothetically dropped onto low-minors rosters in the U.S.

In MLB, most players have to go through several minor league seasons before hitting the majors. For the most part, only future-superstar types debut in their late teens or early 20s.

Where the difference between the two leagues is really felt is on offense. Here's a comparison of the cumulative OPS of 22-and-under players in NPB and MLB over the last 10 years, with data from Baseball-Reference.com and FanGraphs:

Data courtesy of Baseball-Reference.com and FanGraphs.

There have been a couple years in the last decade in which there's been a small gap in hitting quality between the youngsters of both leagues, but NPB youngsters generally haven't been as productive as MLB youngsters. In some cases, the gap has been huge.

This supports the notion that young players water things down in the NPB and, by extension, one of the notions proposed by Keri: that NPB pitchers have it easier. They do indeed benefit from facing young hitters who are out of their depth.

As such, an NPB pitcher has to prepare to face tougher lineups when he makes the jump to MLB. It's no wonder that the track record of former NPB pitchers in MLB is a mixed bag.

Excluding pitchers who didn't pitch in NPB before pitching in the majors (i.e. Boston's Junichi Tazawa), Wikipedia lists 31 pitchers who have made the jump from Japan to the big leagues. Of those, nine have accumulated career WARs of at least 9.0, according to Baseball-Reference.com.

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There have been 11 Japanese imports, however, who have accumulated negative career WARs in MLB. In addition, two of the 9.0-plus WAR guys compiled a career ERA+ at or below 100: Hideo Nomo and Daisuka Matsuzaka. They had their moments, but both technically qualify as at-best average MLB pitchers.

It should be acknowledged that 2013 was something of a banner year for former NPB pitchers in MLB. Yu Darvish, Hiroki Kuroda and Hisashi Iwakuma all had Cy Young-caliber seasons, and nobody could hit Koji Uehara after he took over as Boston's closer midway through the year.

However, none of these four pitchers was particularly dominant in MLB right out of the gate.

Kuroda pitched well in his first three seasons as a Dodger, but his ERA+ in his last three seasons is 12 points higher than his ERA+ from his first three seasons. Darvish had an ERA in the mid-4.00s as late as August in his rookie year. Iwakuma had an ERA near 5.00 in his days as a reliever in 2012. Uehara only had a 4.05 ERA and a 6.5 K/9 in his rookie season in 2009.

As FanGraphs' Dave Cameron recently argued, pitchers from Japan and other Asian countries do need to be taken more seriously in light of the success of recent Asian imports. But for all the statistics that can be bandied about, the writing is still on the wall that going from NPB to MLB means making the necessary adjustments to survive in a harsher league.

As for hitters going from NPB to MLB, well, their track record is even spottier.

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Of the 14 Japanese position players who have made the jump from NPB to MLB, only two have career WARs over 10.0, according to Baseball-Reference.com: Ichiro Suzuki and Hideki Matsui. After them, the most successful Japanese position player in MLB to date is Norichika Aoki, whose MLB career is just two years old.

There's a good reason for the shortage of Japanese position players who have had a legit impact in MLB. For many years, hitters faced a significant adjustment in their own right. Whereas pitchers faced the prospect of pitching to tougher lineups, hitters faced the prospect of hitting a tougher baseball.

You've probably heard about the juiced ball scandal in NPB, one that helped offensive numbers skyrocket and ultimately forced commissioner Ryozo Kato's resignation. But just as important is what happened before the ball was juiced for 2013.

According to the Bangkok Post (by way of Jay Jaffe of SI.com), it was in 2011 that Kato originally called for the league's balls to be altered so they would be more in line with those in the States. This essentially involved making them less springy and, thus, less hitter-friendly.

The change resulted in the desired effect. Too much so, in fact.

With more data courtesy of Baseball-Reference.com and FanGraphs, here's a quick look at the power production of NPB hitters over the last decade, as compared to that of Major League Baseball:

Data courtesy of Baseball-Reference.com and FanGraphs.

The 2004 season notwithstanding, there was already less power to be found in NPB than there was in MLB. Then along came the new baseballs, which sucked power out of the league in a big way.

And that makes sense, as the new baseballs were being unleashed on a population of hitters used to hitting more springy balls. Take away the hitter-friendly balls, and you take away their numbers. Just like what's happened with a good chunk of NPB imports in MLB.

By that same token, however, what also makes sense is that the foreign hitters in NPB would be less impacted by the change. Hitting a more MLB-style ball would be nothing new to them. Consider the following table, which displays how many non-Japanese former MLB players have ranked in the top 25 in OPS in both the Pacific and Central Leagues over the last 10 years:

Former MLB Players' OPS Ranks in Japan Year Pacific League Top 25 Central League Top 25 2013 4 10 2012 5 7 2011 2 8 2010 3 7 2009 8 7 2008 7 3 2007 10 3 2006 9 4 2005 7 4 2004 9 4 Baseball-Reference.com

In the last decade, foreign hitters have tended to account for between 10 and 15 of the best hitters in NPB. That didn't change in 2011 and 2012 when it suddenly got harder to hit the ball, and former MLBers continued to be a force with a juiced ball in 2013.

Notably, former Seattle Mariner and Cincinnati Red Wladimir Balentien set a new single-season home run record in 2013 by launching 60 home runs. After managing a .655 OPS in 170 major league games, he now owns a .989 OPS in 376 NPB games.

Now, Balentien also happens to be an example of one of the key circumstances that makes it hard to compare NPB-to-MLB crossovers to MLB-to-NPB crossovers. Players going from Japan to MLB tend to be in their late 20s or early 30s, meaning they have to make the necessary adjustments while also fighting the effects of age. Players going from MLB to Japan can be much younger, and thus spared the battle against age.

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Balentien was 26 years old in his first season in Japan. He therefore arrived in Japan at an age when, according to research done by Jeff Zimmerman of Beyond the Box Score, MLB hitters tend to hit their primes. The hitters who have come from Japan to the States have tended to be in a different boat.

The tradeoff, however, is one that I mentioned in passing a while back.

The players going from Japan to the majors tend to be among the most talented players in NPB. The players going from the majors to Japan, on the other hand, tend to be guys whose talent has proven to be unfit for MLB. It doesn't reflect well on the quality of the NPB's competition that so many of these players have found success in Japan.

Just look at Casey McGehee. He had a .632 OPS in his last two years in the big leagues. He posted an .891 OPS in his first year in Japan in 2013. Nyjer Morgan has also resuscitated his career, posting a .795 OPS in Japan after posting a .610 OPS in the majors in 2012.

Then there are guys like Tony Blanco and Lastings Milledge. Blanco managed a .490 OPS in just 56 major league games in 2005. He has an .897 OPS in five NPB seasons. After compiling a .723 OPS in parts of six major league seasons, Milledge has compiled an .821 OPS in his two seasons in Japan.

On the pitching front, there are guys like Daniel Cabrera and Brandon Dickson. Cabrera pitched to a 5.10 ERA in the majors, where he was last seen in 2009. In his first year in Japan in 2013, he posted a 3.09 ERA. After being stuck at Triple-A for three years, Dickson posted a 2.77 ERA in his first NPB season in 2013.

Then there are the former first-round busts. Bryan Bullington, the former No. 1 overall draft choice of the Pittsburgh Pirates, has a 2.93 ERA in three NPB seasons. Jason Standridge, the former first-round pick of the Tampa Bay Devil Rays who accumulated a 5.80 ERA in 80 major league appearances, now has a 3.04 ERA over six seasons in Japan.

Not every former MLBer who goes to Japan finds success waiting for him, mind you. Matt Murton's .794 OPS in Japan looks a lot like his .788 OPS in MLB. Ryan Spilborghs only posted a .625 OPS in his first season in Japan in 2013. Casey Fossum had an ERA over 5.00 in his one and only NPB season. Darrell Rasner has a modest 4.17 ERA in Japan. And so on.

There is no question, however, that there are more success stories to be found among the players who have gone from MLB to NPB than there are vice versa. That wouldn't be the case if NPB was as tough as or tougher than MLB.

We know the reasons why it's not. The NPB doesn't filter elite talent into the top level like MLB does, nor can it really hope to. Pitchers benefit from a steady diet of matchups against young, overmatched hitters. As for the hitters, the list of failed NPB-to-MLB imports and the events of 2011 and 2012 clearly suggest that most of them need a more hitter-friendly ball to maximize their offensive potential.

Again, there's no definitive answer to the question of how "good" Nippon Professional Baseball is. And taken for what it is on its own, it's not bad baseball. But due to various circumstances at play, there's no way for it to measure up against Major League Baseball.

So those MLB scouts who view NPB as a Quad-A league? They have the right idea.

Note: Stats courtesy of Baseball-Reference.com unless otherwise noted.

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