Tuesday, August 18, 2009

Advanced Stats (Offense Part 4)

In part 3 we discussed wOBA and how combines both OBP and SLG and uses weights to form a number which resembles an OBP. I then raised the question, what makes wOBA so special? Why is is it better than OPS? The first reason was covered in PART 3 well. A major flaw in OPS is that it treats a player with a .370/.430 hitter the same as a .400/.400 hitter. Both players would have the same OPS (.800) but the latter player is actually a better hitter. This is covered with the use of linear weights methodology.
But clearly, the most amazing thing about wOBA is that it is incredibly simple to convert into runs (aka wRAA, weighted runs above average). You may ask, why convert to runs? I would begin answering this by asking the question what does wOBA really tell you? Sure it's good for evaluating an offensive player's performances but how much is a .380 wOBA worth? The formula for runs weighs wOBA against the rest of the league. So if you see the the league average wOBA is .335 a .380 wOBA will probably be worth a lot of runs. But how many you ask? here is the formula:
((single player wOBA minus league average wOBA divided by scale) multiplied by plate appearances
The formula is pretty self explanatory except for the scale portion. The scale is 1.15, which represents the weights that season are 115 percent greater than the "standards" weights calculated. In doing so, we more closely match that season’s OBP. This scale is only necessary to convert the wOBA figure into runs above average. Yes it's confusing. In my advice don't worry about and just realize that the number is 1.15.
So let's use the example I used before, a player with a .380 wOBA against a league average wOBA of .335, and we will say that he had 600 plate appearances. Let's use the formula:
(.380.335/1.15)*600.
.380-.335=0.045
0.045/1.15=0.0391
0.0391*600= 23.46
wRAA=24.5
So this fictional player used his .380 wOBA to add roughly 24.5 runs to his team during the season. In case you were wondering 24.5 wRAA is good, top 15 in baseball last season.
This concludes the Advanced Stats (Offense) series. Next I will cover defense, replacement, and positions.

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