The statistical impact of switching leagues (for hitters)

Rafael Furcal
If Rafael Furcal signs with the A’s as has been rumored, how would the move to the AL affect his production? (Icon/SMI)

December is here and the rumor mill is churning. It’s getting to be that time when the moves start to come at a faster pace, and with it, we’re bound to see players switching leagues. C.C. Sabathia to the Yankees? Bobby Abreu to the Cubs? Manny Ramirez to the Royals?

Okay… just kidding. We probably won’t be seeing Manny don Royals-blue in 2009, but we are certain to see a number of players jumping from the AL to NL (or visa-versa). When this happens, these players will be facing a different level of competition, which will inevitably affect their performance.

Given this information, does it make sense to project a player without accounting for this new competition level? Fantasy players always seem to be interested in increasing the accuracy of projections, and accounting for league switches is one way we can do this.

How large of an impact does switching leagues have, though, and is the impact larger in some categories than others? I sought to answer these questions, and here’s what I found.


I examined all players who played in one league in 2007 and the other league in 2008. The results I’ll present are the aggregate of all players included in the sample. Each player’s contribution to these results were weighted based on the lower of his 2007 and 2008 at-bat, plate appearance, or other such denominator total. Pitchers were excluded from the batting study. The 2007 numbers were age-adjusted to put them on par with 2008, and both the 2007 and 2008 numbers were park-neutralized. Finally, 2007 numbers were also adjusted for differences in league average.

In the tables presented, the first column gives the total weighted denominator, as explained above. The second column gives the aggregate change simply from switching leagues. These numbers are to be read as if an AL hitter moves to the NL, and you would simply take the inverse for an NL player moving to the AL.

Batting results

Contact rate
Inverse of strikeout rate

+-------+----------+| wAB   | AL -> NL |+-------+----------+| 12993 |  + 1.27% |+-------+----------+


+-------+----------+| wBIP  | AL -> NL |+-------+----------+| 10771 |  + 0.005 |+-------+----------+

Stolen base
success rate

+------+----------+| wSBA | AL -> NL |+------+----------+|  220 |  - 2.04% |+------+----------+


+--------+----------+| wOF FB | AL -> NL |+--------+----------+|   3440 |  + 1.45% |+--------+----------+

Outfield fly ball rate
(OF FB%)

+-------+----------+| wBIP  | AL -> NL |+-------+----------+| 10481 |  - 0.51% |+-------+----------+


+-------+----------+| wAB   | AL -> NL |+-------+----------+| 12993 |  + 0.34% |+-------+----------+

Walk rate

+-------+----------+| wTPA  | AL -> NL |+-------+----------+| 14647 |  + 0.72% |+-------+----------+

Hit-by-pitch rate

+-------+----------+| wTPA  | AL -> NL |+-------+----------+| 14647 |  - 0.02% |+-------+----------+


+-------+----------+| wBIP  | AL -> NL |+-------+----------+| 10696 |  + 0.47% |+-------+----------+


+-------+----------+| wBIP  | AL -> NL |+-------+----------+| 10696 |  + 0.31% |+-------+----------+


+-------+----------+| wBIP  | AL -> NL |+-------+----------+| 10696 |  - 0.11% |+-------+----------+

EDIT: Shortly after publishing, I realized that measuring raw changes in AB/HR, BIP/1B, BIP/2B, and BIP/3B will result in a different effect for each batter. Fixing my methodology, I’ve switched the numerator and denominator in these stats so that effects are uniform for all batters. I’ve also changed the below analysis just a bit to reflect these changes. Sorry for the confusion. –DC 12/1/08


Overall, the general notion that a hitter will perform better in the NL than the AL seems to be true. Most importantly for fantasy owners, a batter gains a full point-and-a-quarter in contact rate, .005 points in BABIP, and 1.5 points in HR/FB simply by playing in the National League.

On the downside, a batter loses two points in stolen base success rate (though it should be noted the sample size for SB% is quite small in relation to every other stat tested) as well as half-a-point in outfield fly rate. Even given the outfield fly loss, though, a move to the NL would still allow a batter to hit an additional two-plus home runs given the big HR/FB spike and the contact rate increase.

Interestingly—though not particularly noteworthy for fantasy owners—is that aside from outfield fly rate (and stolen base rate), the only stat that hitters worsen in is triples.

Concluding thoughts

Keep an eye out later this week or next as I look at how league changes affect pitchers. This could have important ramifications for guys like Sabathia, A.J. Burnett, Derek Lowe, Jake Peavy, and many others.

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