Friday, August 1, 2008

DIPS don't lie

Defensive Independent Pitching Statistics are a very important part of baseball analysis. They take away the issues that are brought about by Wins, ERA, and other commonly used pitching stats. Here is my attempt to use a few stats to break defense away from pitching to reach xERA, the expected ERA based on things the pitcher can control.

The stats I used were IP, H, BB, K, LD%, GB%, and FB%. IP and H were basically used just to find a rough representation of BF. I did this because I was using Fangraphs' export feature to calculate for a long list of data. Here are the steps.

Outs = IP*3

Balls Hit (BH) = Outs - K + H
Basically Ball in Play plus HR. HR are usually seperated but will be dealt with later.

xHits = ((.734*LD%)+(.245*GB%)+(.217*FB%)*BH
The coefficients come from the MLB batting averages on each type of hit type.

xTotalBases = ((1.016*LD%) + (.267*GB%) + (.574*FB%))*BH
Same as xHits, except with SLG instead of BA. This is also where HR are dealt with, as the Slugging Percentages include HR, and HR are mostly a product of GB/FB ratio.

xBA = xH/(Outs + H)
xOBP = (xH+BB)/(Outs + H + BB)
xSLG = xTB/(Outs + H)

Now that we have defensive independent BA/OBP/SLG, we can find out xRunsAllowed. I decided to use GPA because of its simplicity and ability to be converted into Runs.

xGPA = (1.8*xOBP + xSLG)/4
xRA = (Outs+H+BB)*1.356*(xGPA^1.77)

Now that we have xRA, we just have to find out xIP, which is basically xOuts based on xOBP.

xIP = ((Outs+H+BB)*(1-xOBP))/3

Simple. Now xRA/9

xRA/9 = xRA*9/xIP

Since this includes unearned runs, it won't be on the same scale as ERA. Since the difference between ERA and RA is about .360

xERA = xRA/9 - .360

And that's about it.

Here are some results:

All qualified 2008 Pitchers

All qualifed 2007 Pitchers

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