The Impact of Switching Leagues (For Hitters)

July 28 2:53am
Derek Carty

It’s the week of the MLB Trading Deadline, and yesterday we witnessed a flurry of moves involving the Toronto Blue Jays, Chicago White Sox, and St. Louis Cardinals. Leading up to this Sunday’s deadline, we’re likely to see quite a few more trades going down.

While a change of team and environment can have a significant impact on a player’s value, his FanDuel price doesn’t take this into account. FanDuel price is largely calculated using the player’s past performance in his old environment. As such, there is value to be had for players changing teams at the deadline. Today, I wanted to look at one of the most important variables that we need to consider when assessing a player’s new value: league change.

When a player is traded from the National League to the American League (or visa-versa), he is, of course, going to be surrounded by new teammates, new coaches, and new philosophies—essentially, an entirely new environment—but perhaps more important than all of that, he’ll be facing a new level of competition. While you might not initially think that there’s a big difference between the AL and the NL—after all, everyone is in the major leagues, at the highest level of professional baseball in the world—there are some important differences that we can take note of.

Over the next two weeks, I’m going to be examining how the stats of MLB players change when they move between leagues. This week, we’ll look at a plethora of stats for batters, and next week we’ll check out the pitchers.

The Process

To study the effects of changing leagues, I’ve looked at all hitters who played in both the American League and the National League in the same year. Weighting by the shorter of the two stints, I looked at average change in performance for all players between 2000 and 2010. I did not adjust for park, which would likely make a small difference, but this should still provide us with a pretty good gauge. These results are to be read as if an AL hitter is moving to the NL, and you would simply take the inverse if you’re looking at an NL hitter moving to the AL.

The Results

Note: The “Denominator” column lets you know the exact formula used for each stat. So for strikeout rate (K), for example, a hitter moving to the NL will experience a 0.5% increase in his K/(TPA-IBB-HBP).

Overall, we see a very clear trend of hitters performing better in the National League over the American League, all else equal. They tend to strike out a bit more, but they also walk more and draw more hit-by-pitches. Their BABIP increases by about 0.005 points, but their batting average increases by just 0.003 points due to the extra strikeouts. They hit more home runs in the NL thanks to a sizeable 1.4 percent increase in HR/FB, but that is mitigated a bit by a 0.9 percent decrease in the percentage of fly balls. And while hitters tend to get caught stealing more often in the National League, they also attempt steals more frequently, adding up to a net positive in steals.


Overall, the effects aren’t enormous for a hitter moving from one league to the other, but they are definitely there and are worth keeping in mind. If you’re debating hitters for your FanDuel team once the dust settles from the trade deadline next week, maybe it would be worth bumping up your valuation of a hitter whose new home is in the National League by a couple hundred FanDuel dollars.

We do need to remember, however, that this isn’t the only thing changing for a hitter who switches teams. Things like parks and lineup position are also important, so while Colby Rasmus is moving to the tougher league, he will be playing in a very favorable park and could receive a better spot in the lineup than the 6th spot he had been occupying in St. Louis of late.

Concluding Thoughts

That wraps it up for this week. Next week, I’ll be checking out how pitchers fare when they change leagues. As always, if you have any questions, feel free to comment or e-mail me. Also, be sure to add me as a friend on Facebook and follow me on Twitter.

Derek Carty’s work can also be found at Baseball Prospectus, CardRunners Fantasy Baseball, and He has previously had his work published by The Hardball Times, NBC’s Rotoworld, Sports Illustrated, FOX Sports, and USA Today. He is the youngest champion in the history of LABR, the longest-running experts league in existence, and is a graduate of the MLB Scouting Bureau’s Scout Development Program (aka Scout School). He welcomes questions via e-mail, Facebook, or Twitter.

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