Another stat for your viewing pleasure … wOBA

November, 25, 2008
As you've probably noticed, I've become a big fan of FanGraphs, which began life as the home for cool baseball graphs and has since come to rank among the best places for solid analysis and non-traditional statistics. And speaking of the latter, here comes another: wOBA

    Last night, David announced that FanGraphs is officially carrying wOBA as our newest statistical addition. For those of you who have read The Book, you'll be familiar with wOBA, but for those of you who aren't, here's a brief introduction and some reasons why you should give this new, funny sounding stat a try.

    First off, wOBA is a linear weight formula presented as a rate statistic scaled to On Base Percentage. Essentially, what that means is that average wOBA will always equal average OBP for any given year. If you know what the league's OBP is, you know what the league's wOBA is. Usually, league average falls in the .335 range -- it was .332 last year, but offense was down around the game in 2008, which may or may not continue.

    So, why should you care about wOBA? What makes it better than OPS or any of the more famous rate statistics that measure offensive value? The beauty of wOBA lies in linear weights. Essentially, every outcome has a specific run value that is proportional to other outcomes -- a home run is worth a little more than twice as much a single, for instance. What wOBA does, as all linear weights formulas do, is value these outcomes relative to each other so that they are properly valued.

    OPS, as you probably know, significantly undervalues the ability of a hitter to get on base. It treats a .330 OBP/.470 slug as equal to a .400 OBP/.400 slug, when the latter is more conducive to scoring runs. wOBA gives proper weight to all the things a hitter can do to produce value, and is a more accurate reflection of a hitter's value.


    The other great advantage wOBA has is that it's extremely easy to convert into run values. Simply take a player's wOBA difference from the league average, divide by 1.15, and multiply that by how many plate appearances he got, and you have a run value above or below average for that player.

    For instance, using Ramirez, who we already said had a .403 wOBA, which is 72 points higher than the 2008 NL average of .331. 0.072 / 1.15 = 0.063. 0.063 * 700 = 43.82 runs above average.

    wOBA -- league average wOBA divided by 1.15 times plate appearances = runs above average by linear weights. Simple, easy, and accurate. This is the joy of wOBA.

Is wOBA "better" than (for example) Baseball Prospectus' Batting Runs Above Average (BRAA)? According to wOBA, Hanley Ramirez was 48 runs better than average this year; according to BRAA, he was 50 runs better than average. Essentially no difference. But what about Albert Pujols? According to wOBA, Albert Pujols was 67 runs better than average; according to BRAA, he was 82 runs better. That's obviously a significant difference, the sort of difference you'd have to explain if you were being paid to analyze baseball players (and before you say anything, let me remind you that I'm paid not to analyze, but to entertain, like a court jester who occasionally dispenses a bit of wisdom and hopes the result isn't a head-lopping).

You might be too young to remember this, but when I started using OPS in my columns, back in the 1990s, I had to explain it every time. This went on for a couple of years. Then the same thing happened with OPS+, and Win Shares, and Baseball Prospectus' BRAA and WARP (Wins Above Replacement Player). I don't mind going through the same process with wOBA, especially because explanations are easier when you can just explain with a link. But I'm sort of flying blind here.

What I like about wOBA is that it's relatively transparent and is translatable to runs. OPS is transparent but doesn't look like anything; OPS+ is neither transparent nor familiar. What I don't like about wOBA is the nomenclature. It's weighted on-base average … except it's not, at all. It's really linear weights on a scale that looks like on-base percentage, just like BP's Equivalent Average (EqA) is on a scale that looks sort of like batting average. It's problematic for me to use a statistic that's got a name suggesting something that's not true. Also, it appears that wOBA is not park-adjusted.

Meanwhile, the Baseball Prospectus metrics are -- last I checked, anyway -- completely opaque. Because BP is a for-profit enterprise that holds its intellectual property closely, we're just supposed to trust them.

Which I do, generally. But that science-vs.-enterprise dynamic can be tricky. The methodology behind BP's metrics is not, to my knowledge, peer-reviewed. If one or two people make a big mistake, would anyone else know? Now, let's jump ahead and say that two or three years down the line, the big mistake was discovered internally. Would BP announce to the world that all those numbers over the previous three years had been wrong? Or would the guys running the show decide that the loss of credibility (and potentially, revenues) isn't balanced by the loss of integrity?

This is not merely a thought experiment. I once witnessed a similar situation, and the decision was in favor of credibility rather than honesty. I like and respect the people who run Baseball Prospectus. But like the rest of us, they want to make good livings. Which does give me minor pause. Science works best under blue skies, with little thought of green.

Meanwhile, there's yet one more issue … What I usually want to know isn't how good a hitter someone is. What I really want to know is how good a player he is, and WARP, by combining hitting and fielding, tells us this. We're still waiting for someone to incorporate (non-steal) baserunning, but the data's there and I suspect the wait won't be long. That's really what I want to see: a comprehensive, open-source metric that includes everything we can measure. Really, it's sort of amazing that it doesn't exist yet (at least not publicly; I'm sure a number of teams are doing this already).

So, what do you think? Should I add wOBA to the toolbox? Or are you getting enough already…



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