So what are OBP/SLG/OPS/wOBA anyway?

On the heels of the seminal For All The Dummies fanpost and the excellent contributions on FIP and statistical education in general, I thought I'd do my part by explaining some basic and advanced hitting stats.  Many of you will already understand much of this, so I'll just go from the basics that we all know to what I think is the best single hitting stat.

Good ol' Batting Average and its problems.  We all grew up knowing that a player who hit .300 was a good hitter.  So what's wrong with using batting average to evaluate a hitter?  The problem is that batting average misses two key things.  First, it only covers hits.  There are other important ways to get on base too, like Walks.  Second, it equates all hits.  A single is not as valuable as a double or home run.

Partial solutions: OBP, SLG and OPS.  So one can turn to two other stats which include hits, but fill in the gaps that batting average leaves.  On Base Percentage (OBP) [sometimes referred to as On Base Average or OBA] describes how often a batter reached base.  It is calculated by adding hits, walks and hit by pitches and dividing that by total plate appearances (not just at bats).  Slugging Percentage (SLG) attempts to measure the relative value of a player's hits by dividing Total Bases (4 for a home run, 3 for a triple, etc.)  by at bats.  Adding those two together gives you On Base Plus Slugging Percentage(OPS) which is a handy, but clumsy single hitting stat.

So what's wrong with OPS?  The main problem is that it equates OBP and SLG when they aren't equal.  First, they are on different scales: .000-1.000 for OBP and .000-4.000 for SLG.  Second, and more importantly, OBP and SLG don't equally contribute to run scoring.  In short, a raft of studies have found that OBP is much more important. 

Problem solved: wOBA.  Preeminent sabermetrician Tom Tango created a stat called Weighted On Base Average (wOBA) to appropriately account for the value of getting on base, hitting for power and pretty much all other outcomes from a plate appearance.  This metric uses a system called linear weights to determine the run value of the various outcomes of a plate appearance.  Without going to into too much detail, this is an empirical approach which looks at how various events (like a double, a walk or a stolen base) have affected run scoring over multiple years of data.  [If you'd like a more detailed, math-heavy explanation of linear weights, click here.]  Calculating wOBA is a bit complicated, so I let others do it, but it basically involves multiplying the run value coefficient of each event (like a double) by the number of times the player did that (how many doubles he hit), adding up all of those results for all events and dividing it by total plate appearances.

What does this number mean?  This stat is scaled to OBP, so if you know what an average or good OBP is, then you know what an average or good wOBA is.  As Dave Cameron at Fangraphs put it:

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.

What doesn't wOBA include?  It isn't currently park normalized.  It's like standard, raw rate stats in that guys in hitter's parks are going to do better and players at pitcher's parks are going to do worse.  Also it doesn't attempt to equalize players at different positions.  There are metrics which recognize the different real world hitting standards for a shortstop as compared to a first baseman, but wOBA isn't one of them.  But they exist.  We'll save that for another post in this series.

So how does this apply to the Royals?  Here is the Royals 2008 wOBA leader board (min. 100 PA):

Mike Aviles .360

If you are interested in learning more about wOBA and some other advanced hitting metrics, I would suggest the following sources, many of which I used to put this together:

What's new at Fangraphs, by Eric Seidman at Hardballtimes.com

The Joy of wOBA, by Dave Cameron at Fangraphs.com

Linear Weights, by Tom Tango at Tangotiger.net

The Book: Playing the Percentages in Baseball, by Tom Tango, Mitchel Lichtman and Andrew Dolphin [This is a book, so you'll have to buy it.  No internet freebies.]