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Impact of Pitchers Changing Leagues

Every year around this time, we hear baseball experts talk about how pitchers can expect to benefit or be harmed by switching leagues, but how much of an effect is there really?

This year there seems to be a mass exodus of big-name pitchers moving to the Senior Circuit—Cliff Lee, Zack Greinke, Shaun Marcum, Matt Garza and Javier Vazquez to name a few—so how much should we expect their stats to improve? In fantasy baseball, does this vault them up our preseason rankings or should we only value the switch as a tie-breaker between otherwise even candidates?

To answer this, I went back and looked at the ERA and K/9 for American League pitchers and National League pitchers over the last 10 seasons (since these are the two most telling and fantasy-relevant stats) to see how large the AL-NL gap has become.

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Is it surprising to see that in each of the last 10 seasons the NL has had a lower ERA and a higher K/9 than the AL? Actually, yeah, I am kind of surprised. Just as we expected, the 10-year averages for ERA and K/9 certainly favor the NL, but for it to be tilted in the NL’s favor in every single season over the last decade is notable.

So, what do these differences equate to for one pitcher over the course of a season? The difference in ERA between leagues is straight-forward; NL pitchers averaged an ERA that was 18 points lower than AL pitchers. But what about strikeout totals? For a pitcher who throws 200 innings (and many of the pitchers we care about in fantasy baseball are 200-inning beasts) the difference is only eight strikeouts—144 per 200 IP in the AL and 152 per 200 IP in the NL.

Honestly, I thought the gap would be a little larger, but if you combine the drop in ERA and the slight rise in strikeouts and K/9, you do get a better overall line. Does this change the way I value a pitcher like Marcum or Garza? Absolutely not. Because these numbers only take into account one league versus the other, division strength is not accounted for. Marcum and Garza’s moves from the AL East to the NL Central represent drastic changes on two fronts; not only are they leaving the AL, but they’re also leaving the AL East. Both of these factors combine to drive up their values.

Greinke, on the other hand, might not notice that much of a difference. The AL Central isn’t exactly a power division, and it could be argued the NL Central houses better offenses with teams such as St. Louis and Cincinnati. Unlike Marcum, Greinke only gets a boost by changing leagues and not by leaving the AL Central.


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