Check out the last slide of the Decision Tree, Event Tree thread that I bumped.  It shows the handedness modifier for a RHP/LHB (Pedro/Ruth) matchup to be 1.045.  So about a  4.5% "bonus" to the hitter when facing an opposite-handed pitcher.

Skunk thanks for clarifying. I obviously misread what contrarian wrote.

so given that there are more RHB than LHB a LHP would be penalized more often than a RHP creating a natural bias against LHP which is what Biglen and Radlynch suggests...and it explains partially why Spahn is underperforming.

Exactly....

Now think about real life....

LH hitters have a problem, in general, hitting against LHP.  So they mostly sit against dominant lefties. 

Randy Johnson was perhaps the most dominant LHP of his generation...
In two prime seasons:
2000 - 248 IP - 906 RHB PA, 95 LHB PA.... 10-1 ratio... overall WHIP 1.12
2001 - 249 IP - 786 RHB PA, 103 LHB PA.  7.5 - 1 ration.  Overall WHIP 1.01

Using the sim's method of calculating, if he faced the same batters at the same ratio, he'd have a 1.17 WHIP in 2000 and a 1.055 ration in 2001

Similarly, Greg Maddux in 2000 had a 1.07 ratio.  RHB had 541 PA and LHB had 431 PA against him.  In this sim his real life 1.07 WHIP would only be 1.06... CLOSE but not quite what he should do.  

So LHP are penalized for being LH... They face a majority of RHB, yet in the sim, their stats vs RHB deteriorate by 5% anyway. 

If you choose a LHP, you're essentially adding 3-5 point to his WHIP.  And that's assuming that there is no additional error (which I think there is) in dealing with LHP.

biglenr has nailed it...in effect WIS is "doublecounting" the platoon advantage against LHP.

I would point out that they also double count park effects....take a real life player from Coors...his offensive stats are no doubt inflated from playing there.  Put him in Coors on a WIS team, and they inflate again. 

I tried arguing this a few years ago with Tom Z...   It's the same thing with HR from high BB hitters.  The order of the way they calculate suppresses the HR total from players with high walk totals.  It's not obvious with guys like Ruth, who are so far above their peers in HR/100AB, but with modern players like Bonds, Frank Thomas, etc, it's very obvious. 

Thank you...I've learned something that heretofore had not occurred to me.

Ok, so I investigated, partially, my assumption that RHP are more advantaged. But I only looked at the top 25 RH and 25 LH pitchers with 300+ innings who played 10 or more SIM seasons, and compared real life WHIP and opponent average versus SIM WHIP and OAVE.  And, turns out, for this group, I am incorrect -- they are about the same, and there is a slight advantage for LHP...

Now, because I didn't use a matching, case control method, the results aren't perfect, and they don't extend beyond the top 25 for each handedness . . . .
But, RHP had a SIM WHIP 37% higher than real life, while LHP has a SIM WHIP 34% higher than RL.  Also, RHP had a SIM batting average against 25% higher, while it was 22% higher for LHP.

Now, this doesn't mean you should go out and draft LHP because the average real life WHIP and average is higher for LHP.  But, at least among this sample, the bias I though existed, doesn't.  If I am motivated, maybe I'll try this for some other groups of pitchers......

Compare to pitchers in the same year.  Normalization is a much bigger factor than handedness

Ok...so following Biglenr's suggestion, I did the same calculation for one year and used pitchers from 1968.  Now, to get a relevant sample of starters, I used 150 IP as the cutoff, and pulled the top 22 RHP and LHPs (limited to 22 because there were only 22 LHP who tossed 150 + IP).  The results aren't much different.
Average SIM WHIP for RHP was 34.5% above real WHIP; Average SIM WHIP for LHP was 33.8% higher than real WHIP.  With respect to opponent average, RHP were + 23.5% above real opponent average, and LHP were 23.7% above.

So, while this is only 2 instances (top 25 LH and RH pitchers, and 1968 starters), there is no big difference between LHP and RHP.

Now, the difference is that LHP tend to be weaker overall than RHP, so they start with higher average WHIPS and OAVE, but the percent difference isn't significant between RHP and LHP.

I did a mini experiment taking 1968 Bob Gibson and 1968 Steve Carlton and had them pitch 10 games each against the same 2004 Boston Red Sox. I figured I would control for the opponent so as to control for normalization as much as possible. Gibson's real life numbers are 1.12 ERA and .85 WHIP. His sim results were 1.69 ERA and .92 WHIP....as for Carlton his real life numbers are 2.99 ERA and 1.19 WHIP. His sim numbers were 5.59 ERA and 1.56 WHIP. My conclusion is Gibson's variance is due to sample size and normalization. Carlton's due to sample size, normalization and some kind of lefty bias that exaggerated his numbers.

Posted by biglenr on 10/17/2013 2:24:00 PM (view original):
I tried arguing this a few years ago with Tom Z...   It's the same thing with HR from high BB hitters.  The order of the way they calculate suppresses the HR total from players with high walk totals.  It's not obvious with guys like Ruth, who are so far above their peers in HR/100AB, but with modern players like Bonds, Frank Thomas, etc, it's very obvious. 

r u saying the HR totals are suppressed or the HR/AB for those high BB hitters is suppressed?  IF everything is programmed correctly, the calculation order shouldn't matter.