Comparing NL pitchers to AL pitchers

[emmett16]

I've always been curious about comparing NL pitchers vs. AL pitchers.   Game 1 had Sale vs. Kershaw and all talk radio was comparing theirs stats side by side.   Does anyone know how to run a search on Pitchers stats. vs. Pitchers hitting?   I've always thought the NL pitchers had an easier time through a line up.  Is there a way to pull their stats vs. pitchers to make side by side stats more comparable.  IE.  Kershaw has a 2.39 lifetime era.  What would his ERA be if you take away all stats vs. an opposing pitcher?

[Frobby]

I don’t think this calculation is really possible, because runs result from a sequence of batters and in the NL the pitcher is just one batter in that sequence.   You could figure out opponents’ batting average or opponents’ OPS with the pitchers excluded, but not ERA.

FWIW, pitchers in the NL had 5.1% of all plate appearances and batted .116 with a .295 OPS in 2018.    Overall, NL batters had a .247 BA and a .722 OPS.    The non-pitchers had a .254 BA and approximately a  .745 OPS.   Finger to the wind calculation, that probably equates to about .25-.30 runs/game lower ERA due to the pitchers.   But you can’t really give an exact number.   

[emmett16]

Thanks a lot, Frobby.  That all makes sense.  I didn't even think about the era calc as being multiple batters at first.  The 5.1% number is interesting - I assumed it would be somewhere around 2.25/27 but was way off.  I appreciate the leg work on that.  I find it interesting that this isn't a commonly addressed when analysts are comparing NL v AL pitchers.   When I heard that Game 1 was Kershaw's first career appearance vs. the Red Sox I started thinking about how his #'s would stand up in the AL.  Thanks again!

[SurhoffRules]

18 hours ago, Frobby said:

I don’t think this calculation is really possible, because runs result from a sequence of batters and in the NL the pitcher is just one batter in that sequence.   You could figure out opponents’ batting average or opponents’ OPS with the pitchers excluded, but not ERA.

FWIW, pitchers in the NL had 5.1% of all plate appearances and batted .116 with a .295 OPS in 2018.    Overall, NL batters had a .247 BA and a .722 OPS.    The non-pitchers had a .254 BA and approximately a  .745 OPS.   Finger to the wind calculation, that probably equates to about .25-.30 runs/game lower ERA due to the pitchers.   But you can’t really give an exact number.   

It's hard to make an apples to apples comparison, but if we assumed that the AL would have to let the pitcher bat, they one way to estimate it might be replacing the 9th place hitter for rough 2/3s of their at bats.

In 2018 the 9th place AL hitters slashed .227/.285/.353/.638 in 9172 PA.  If I replace an equivalent number of those at bat with the NL pitchers production, the AL all-hitters line would drop to: .243/.312/.406/.718. For reference the NL all-hitters line is .247/.318/.403/.722.  (FWIW, the NL had about 500 more PAs than the AL).

Like you stated, there's no way to tell how making the pitcher a hitter would impact the rest of your lineup and who gets those PH at bats, but if you're willing to entertain my assumptions the difference in production between the "no DH leagues" might not have been all that great in 2018.

Edit: I recognize this is a question about pitching ERA, but it got me thinking.