Several years ago, Tom Tango introduced the Weighted-On-Base-Average (wOBA) statistic in The Book: Playing the Percentages in Baseball. Not long after that, wOBA was added to the FanGraphs statistics database. The wOBA measure hasn't become as popular as On Base Plus Slugging (OPS), but it is no longer an obscure statistic used only by hardcore stat guys.
If you spend a lot of time reading about and discussing baseball online (I assume that's most of you here), it's kind of hard to avoid wOBA. I use it a lot here. The Bless You Boys and and Motor City Bengals bloggers mention it frequently. You see it on the MotownSports forum and Twitter. It's all over FanGraphs of course. It even shows up on mainstream channels like MLB and ESPN on occasion. Despite its growing popularity, I think a lot of people still don't have a great grasp of what wOBA is or how it works, so the annual primer seems worth it.
The wOBA statistic is like an on-base-percentage (OBP), except that it gives appropriate weights to different events. As you know, the OBP calculation counts every event where a batter reaches base (walk single, double, etc) the same. In contrast, wOBA gives a hitter more credit for a hit than a walk and more credit for doubles, triples and home runs than singles. The result is a rate statistic which measures a players total batting contribution.
One of the great things about wOBA is that it is scaled to behave like OBP. So, we know that .375 or better is very good, .325 is about average for a starter or semi-regular, and below .300 is poor. The top wOBA for the Tigers this year is Victor Martinez at .417. We know that an OBP of .417 would be outstanding. A wOBA of .417 is equally outstanding, but it measures Martinez's overall batting contribution rather than just his ability to get on base. Andrew Romine, on the other hand, has a wOBA of .239. We know that a .239 OBP is horrible and a .239 wOBA is equally horrible.
Why not OPS?
Why can't we just use OPS? The problem with OPS is that OBP contributes about 80% more to run scoring than slugging average (SLG). Since OBP and SLG carry equal weight in the OPS formula, this means that OPS undervalues OBP relative to SLG. Since wOBA weights events more appropriately, it is a better reflection of a player's total batting contribution. OPS is a decent measure of a player's overall batting performance and we don't need to abandon it entirely, but wOBA is a better alternative when we want to be more precise.
wOBA versus OPS
Table 1 below shows where Tigers rank among American League hitters with at least 130 plate appearances in 2014. You can see that Martinez leads the league in both wOBA and OPS and that most players rank similarly on the two measures. The biggest discrepancy on the Tigers is catcher Alex Avila who ranks 33rd on wOBA and 40th on OPS. The reason for the less favorable rank on OPS is because OPS does not give Avila enough credit for his best offensive skill - getting on base. The lesson to be learned here is that OPS usually works pretty well, but that wOBA gives you a more accurate assessment of some players.
Table 1: Tigers Ranks in wOBA and OPS
Player | wOBA | wOBA Rank | OPS | OPS Rank |
Martinez | .417 | 1 | 1.004 | 1 |
Cabrera | .387 | 8 | .902 | 9 |
Kinsler | .366 | 20 | .837 | 21 |
Avila | .348 | 33 | .772 | 40 |
Hunter | .333 | 44 | .761 | 44 |
Davis | .330 | 48 | .746 | 50 |
Jackson | .328 | 52 | .753 | 48 |
Median | .326 | 56 | .732 | 56 |
Castellanos | .279 | 101 | .635 | 97 |
If you don't like math, you can stop here hopefully with a better understanding of wOBA. If you want to see the details, read on.
Calculation
In order to calculate wOBA, we need to consider the weight or run value of each event relative to the weight for an out. We know how much each event is worth by looking at all kinds of situations over thousands of games. For example, a home run is worth 1.64 runs more than an out on average. The weights for each event are as follows:
1B 0.69
2B 0.99
3B 1.26
HR 1.64
BB 0.54 (intentional walks excluded)
HBP 0.56
We now have a new formula:
Run Rate = (0.69 x 1B + 0.99 x 2B + 1.26 x 3B + 1.64 x HR + 0.54 x BB + 0.56 x HBP)/(PA-IBB)
The MLB average run rate is .244 per plate appearance relative to the run value of an out for all batters in 2014. We could stop there, but in order to be on the same scale as OBP we want average wOBA to be about .317, the league average OBP for everyone (not just guys with 130+ PA). Now, 317 is 29.9% higher than .244, so we multiply all of our weights by 1.299 and arrive at the following formula:
wOBA= (0.90 x 1B + 1.29 x 2B + 1.64 x 3B + 2.13 x HR + 0.70 x BB + 0.73x HBP)/(PA-IBB)
Note that FanGraphs excludes intentional walks from wOBA because they are usually issued in very specific situations and many analysts feel as if they have as much to do with game situation as with player value.
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