As a follow up to the first Better Know a Stat entry which explained OPS+, wOBA is an advanced statistic which gives different weights to on base percentage and to slugging percentage, as opposed to OPS gives equal weight to the two.
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
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.
Simply speaking, the ability to get on base is the most important quality a hitter can have due to the rules of baseball. To paraphrase from Moneyball, unlike any other American sport, baseball is an un-timed game. So long as there are additional outs, a team is never out of the contest. As such, those outs are the most precious resource the hitting team has. With every out, the game is shortened, and every time a batter leaves the batters box without an out being recorded they have extended the game.
The fact that OBP is more important than SLG seems to be fairly instinctual within the TBLA community, and perhaps the player that most people point out the failings of OPS is when discussing Russell Martin. Martin certainly had a rough stretch at the plate last year, and his OPS fell from .781 in 2008 to .680 in 2009. Suffice it to say, an OPS of .680 is pretty bad for a catcher-- actually its pretty bad for basically any position. In fact, it ranks 31st out of 45 catchers with at least 200 plate appearances in 2009. However, Martin still posted a quite respectable OBP of .352 (good for 8th amongst catchers in 2009 with at least 200 plate appearances) so the most damaging aspect of his OPS was the very poor SLG of .329 (37th out of 45!) As such, wOBA provides a useful tool when measuring Martin who has such a sharp split with how he stacks up amongst his peers. Once giving more weight to OBP, as wOBA does, Martin posted a wOBA of .307, which is, frankly, still pretty bad (26th out of 45.)
Looking towards 2010 for Martin, one could argue that absent improvements in his SLG, he will have a hard time maintaining his stellar OBP. By posting a SLG less than his OBP, pitchers will be more inclined to challenge Martin as 2009 showed that Martin is having a hard time hitting for power. That’s not to say that 2010 will be the same struggles for Martin of course, as their have been reports that Martin added 20 pounds of muscle in the offseason. But wOBA should expose the fallacy that Martin’s strong OBP should forgive his inability to hit for power. One last and somewhat surprising note, Martin’s wOBA of .307 in 2009 compares unfavorably to favorite TBLA whipping boy Juan Pierre’s career wOBA of .325.