Weighted On Base Average or wOBA

On-base percentage is a great statistic because it tells you something important, and in a clear language: at what rate did this player reach base? It doesn’t tell you how far he reached base (second base? third? home?), but only whether he did or did not.

Slugging percentage is another great statistic because it tells you something important, and in a clear language: how many bases did the batter gain for himself per at-bat? It doesn’t consider walks as either a positive or negative event (it simply strips them away as if they don’t exist). It also tries to establish the importance of the single and HR by weighting the HR four times as much as the single.

We have one statistic that is deficient in one area, and another one that is deficient in another. Why not simply combine them as: OBP plus SLG, and call it this new-age statistic named OPS? Might this statistic allow the deficiencies in OBP and SLG to cancel each other out? Let’s see.

From the preceding section, we know the run values of each event. For example, we know that the run value of the HR is 1.4 runs above average, and 1.7 runs above the run value of the out. In rate measures, like OBP, the value of the out in the numerator is zero. If we recast the run values of the most common events relative to the out (rather than relative to the result of an average plate appearance), we get the following:

HR 1.70, 3B 1.37, 2B 1.08, 1B 0.77, NIBB 0.62.

Those numbers are the values of each of our events (again, relative to an out, which now has a value of zero). If we apply these weights to the statistics of a league-average hitter, and divide by plate appearances, we end up with a rate of almost 0.300. This is a fairly convenient number for an average, but we can do better. Since we like OBP as a measure of a batter’s effectiveness, let’s scale our new statistic so that the resulting values are similar to OBP values. It turns out that, if we add 15% to this 0.300 figure, we get the league-average OBP. Therefore, we will add 15% to the weights of each event and define our new statistic as follows:

(0.72xNIBB + 0.75xHBP + 0.90x1B + 0.92xRBOE + 1.24x2B + 1.56x3B + 1.95xHR) / PA

Note: Depending on the specific analysis, the PA term (plate appearances) may exclude bunts, IBB, and a few of the more obscure plays.

Do we really need another statistic? Yes, we do. Instead of trying to take two statistics (OBP, SLG) and combine and correct their flaws in the hopes of getting one number, we prefer to start from scratch. Furthermore, by recasting the number onto the OBP scale, it makes it much easier for the reader to get a grasp on the number. wOBA is weighted on-base average (we call it an average rather than a percentage). When you look at wOBA numbers throughout the book, just think OBP, and you’ll be fine. In other words, an average hitter is around 0.340 or so, a great hitter is 0.400 or higher, and a poor hitter would be under 0.300.

If you are a little more experienced with run values, you might have figured out the following:

Run value per PA above average = (wOBA for player - wOBA for league) / 1.15

So the run value chart, which we presented in the previous section, and the wOBA statistic defined in this section are directly related.