Monday, May 26, 2014

What is wOBA?

I'm seeing a lot of talk about "wOBA" on twitter this morning, so it's time for my annual wOBA primer.
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
Data source: FanGraphs.com

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.

7 comments:

  1. Another great statistic, but unfortunately it doesn't have some of the advantages of other variables like what the RE24 stat includes. The name of the game is to understand all of the stats as well as to bridge and marry them the right way to come to an overall conclusion about each player.

    Studying the numbers can get very math intensive, so to me the science of evaluating players is a form of math art!

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  2. You know what really struck me about the list of Tiger wOBAs? We have no particularly strong offensive outfielders--they are all just a tad above average, which I am sure means that our corner outfielders are far less productive than an average player at the same position. And I have very little confidence that Davis will tread water at his current level as the season goes along, given his career-long struggles versus RH pitching.

    ReplyDelete
    Replies
    1. I agree, I also wish we had one of the premier SSs in the game since they are rather rare at present time. I was hoping for the last several years that we would find a way to nab somebody really special.

      And that's part of the reason why I didn't care for the Hunter signing at the time. It was just a short-term play at a relatively stiff rate with no chance for a long-term parlay or good exit strategy via trade. To me he is a fine ballplayer, but he was also a distraction from trying to build up more young OF stars. Technically we could benefit from having 3 new OFs emerge in the near future, so we have the spots to fill, just need to find the gem value players, and not the poorly leveraged choices.

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    2. And same goes with the Fielder trade. Kinsler is great, but I would have rather displaced PF's trade value for somebody younger and cheaper, or combined him with a guy like AJ or done other trades to package together and get something more interesting.

      Kinsler has been great so far. Heck if I was the GM as of today I'd be getting on the phone and finding out what we could get for Kinsler. You might say well gee that sucks what are we going to do at 2B?! I would say well sucks to be us at 2B for this moment in time because I have to know what I can get for Kinsler. I might not be able to move AJ now because I think we can agree his stock price is lower than it was before the season started.

      So first thing I would say is well thanks to DD we lucked out on Kinsler so far and maybe we can get more in trade for Kinsler today than we could have for PF when we traded PF, so great for us if that's the case! But nevertheless while it may seem inconvenient to trade Kinsler now, to me it's still necessary to always have a strong gauge on the long-term outlook and if we can move him to secure a more strategic piece, and maybe even package him with something other than AJ, well then that needs to be considered as an option. And then there would eventually be a way to circle back to addressing the 2B position for the long-term as well, and so forth and so forth for the various non-pitcher roster spots.

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    3. "I would have rather displaced PF's trade value for somebody younger and cheaper," Well, sure. And I would love to trade Phil Coke for Mike Trout. But we were incredibly lucky to get a great ballplayer for Fielder, saddled as he is with his absurd contract. Frankly, I would have been quite happy if we could have traded Fielder (and his contract) even-up for nothing. DD pulled off a miracle with that trade.

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    4. Well what you are joking about is getting a better player for a lesser player, that's not going to happen so it's not worth thinking about and that's not what I was suggesting. I didn't say we should trade PF for some player that we can't get for PF, I was merely talking about the asset class of the type of player we were to trade for. We could get 10 prospects for PF if they were all the worst prospects in the world that had literally 0 chance of ever playing pro ball. The more valuable the younger/cheaper player is, then the harder it is to get that player. So depending who the specific player is that is younger and cheaper than Kinsler, we may have had to use up more resources in addition to PF in order to get enough to get that one strategic piece. I felt we should have done more cycling out of a lot of players and with AJ's stock being higher before the season started, well if we give up PF and AJ then naturally we can get a much better prospect than if we just traded PF by himself. To me this is a matter of packaging more players together, or doing more trades that in aggregate after several steps net us the best target we can get.

      And as said before, if we can get more for Kinsler today than we could have got for PF at the time we traded PF, then that's even better for us in that it makes it easier for us to convert Kinsler's value towards a younger/cheaper target.

      Delete
  3. Yup, that was one of DD's best moves ever. I couldn't believe they were able to move Fielder's contract and get a good player in the same deal.

    ReplyDelete

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