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 onbasepercentage (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 semiregular, and below .300 is poor. The top wOBA for the Tigers last year was Miguel Cabrera at .417. We know that an OBP of .417 would be outstanding. A wOBA of .417 is equally outstanding, but it measures Cabrera's overall batting contribution rather than just his ability to get on base. Ramon Santiago, on the other hand, had a wOBA of .253. We know that a .253 OBP is horrible and a .253 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.
Percentiles
Table 1 below shows the percentiles for wOBA with 250 or more plate appearances (PA) in 2012. That includes all regulars and parttimers who were considered good enough to see semiregular playing time. For example, the 75th percentile for wOBA is .352. This means that 75 percent of players with 250+ PA hit below .352 and 25 percent hit better than that. Since wOBA is listed along side the more familiar OPS and BA, the chart should help some understand it a little better. For instance, we can see that a .352 wOBA is about equivalent to a .815 OPS or .287 BA.
Table 1: wOBA Percentiles, 2012
PCTL

wOBA

OPS

BA

100

.438

1.041

.346

90

.376

.877

.304

75

.352

.815

.287

50

.325

.743

.261

25

.299

.684

.239

10

.279

.630

.222

0

.243

.530

.176

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 1.65 runs more than an out on average. The weights for each event are as follows:
1B 0.71
2B 1.01
3B 1.28
HR 1.65
BB 0.56 (intentional walks excluded)
HBP 0.58
We now have a new formula:
Run Rate = (0.71 x 1B + 1.01 x 2B + 1.28 x 3B + 1.65 x HR + 0.56 x BB + 0.58 x HBP)/(PAIBB)
The MLB average run rate was .256 per plate appearance for all batters in 2012. We could stop there, but in order to be on the same scale as OBP we want average wOBA to be about .319, the league average OBP for everyone (not just guys with 250+ PA). Now, 319 is 24.5% higher than .256, so we multiply all of our weights by 1.245 and arrive at the following formula:
wOBA= (0.88 x 1B + 1.26 x 2B + 1.59 x 3B + 2.06 x HR + 0.69 x BB + 0.72x HBP)/(PAIBB)
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.
Results
Table 2 below shows the wOBAs for some past and present Tigers. The final column shows specific percentiles. For example, Austin Jackson's .371 wOBA fell at the 87th percentile of all batters with 250 or more PA. That means he hit better than 87% of MLB players in 2012 without regard for position. Other Tigers with high percentiles were Cabrera (99), Prince Fielder (97) and Andy Dirks (86). The lowest percentiles were Ramon Santiago (2), Ryan Raburn (2) and Brennan Boesch (17).
Table 2: wOBA for Tigers, 2012
Player

PA

wOBA

PCTL

Cabrera

697

.417

99

Fielder

690

.398

97

Jackson

617

.371

87

Dirks

344

.368

86

Hunter

584

.356

79

Avila

434

.327

54

Infante

588

.310

36

Berry

330

.305

32

Young

608

.305

32

Peralta

585

.301

27

Boesch

503

.288

17

Raburn

229

.256

2

Santiago

259

.253

2
