Should Dioner Navarro Continue to Switch-Hit?

In an effort to continue to educate our community in sabermetrics, I thought I would demonstrate how to estimate the true platoon skill for Dioner Navarro through regression. Warning: this is pretty number heavy and I am certainly not the caliber teacher that Professor Slowinski is in demonstrating topics clearly. Nonetheless, its a valuable exercise. Please feel free to ask questions.

Dioner Navarro's 2009 offensive struggles have been well-documented. Aside from his difficult season, Navarro also posesses a wide career wOBA (weighted on-base average)  split for a switch-hitter (.283 vs RHP, .334 vs. LHP). With right-handed masher Kelly Shoppachnow in town, Navarro faces the prospect of hitting against mostly RHP. This raises the question: Would Dioner Navarro be better off batting right-handed exclusively?

While intuition says what could be the harm in trying to bat right-handed with a platoon split of .051 (.334-.283), we need to utilize expected regression to get a better estimation of his true platoon split. In The Book, Tom Tango says we only need to regress vs. 600 PA of average platoon skill. Tango calculates the average platoon skill of a switch-hitter to be approximately zero.

Based on our estimate of how much to regress, a player with around 600 appearances against left-handed pitchers should regress about halfway toward the league-average.

Navi has 478 career plate appearances against LHP where his wOBA is .051 higher than versus RHP.  We will need to regress that margin by 55.6% (600/(478+600)).  If we take Navi's margin of .051 and multiply it by (1-55.6%) we get .023. This falls slightly within typical switch-hitter variation of .025.

Navi has a career wOBA of .297 with 478 of his 1794  plate appearances coming against left-handed pitching (26.6%). 

Now we can do this:

.297= (26.6% * (x +.023)) +73.4%x  or

Career wOBA= (%PAvsLHP * vsLHPsplit) + (%PAvsRHP * vsRHPsplit)

When we solve for x we find out that x=.291 which is Navi's regressed wOBA vs RHP.

That leaves us with x+.023 = .314 wOBA vs LHP. The average split margin for right-handed batters is .017.

The truth is Dioner Navarro is not as bad against right-handed pitching as we thought. Sadly it makes him  less useful against left-handed pitching by a bigger margin. The reality is that Dioner Navarro is simply not a very good hitter.

 Note: Switch-hitters are the easiest to regressions to run as the average platoon skill is zero. If interested, we can walk through a lefty and righty in a different post.