Intro to Sabermetrics: wOBA
Over the early part of this season we’ll be rolling out a series of posts that helps explain some of the advanced statistics we use here at BSL. In order to do that we’ll be using a familiar lens: the Orioles. These first few posts will primarily look at last year’s numbers, but as we get into the season we’ll start comparing 2015 stats to 2014 version to see how players are improving or regressing.
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For the first post, I wanted to start with something that’s a building block for understanding overall offensive value: wOBA. wOBA is short for Weighted On-Base Average and was invented by Tom Tango, a noted saberist and current consultant for the Chicago Cubs. At it’s heart, wOBA follows the principal that not all hits are equal (batting average) and not all ways of reaching base are equal either (on base percentage). Furthermore, the weighting of slugging percentage (4*HR+3*3B+2*2B+1B) does not accurately reflect the likelihood of each event to score runs. As such, wOBA uses league-derived constants for the number of runs that each event has caused during that MLB season. This all sounds very complicated, but it’s not. Let’s look at an example from the 2014 season to see wOBA in action:
Last season each time a batter walked they added 0.689 runs for their team. So if a batter walks, they should get credit for that many runs since that’s how many they theoretically added to their team’s total. This makes wOBA more accurate than those alternative options because it uses actual results to connect an action on the field with the number of runs that action should generate.
We can repeat this process for hitting a single (0.892 runs), hitting a double (1.283 runs), hitting a triple (1.635 runs), and hitting a home run (2.135) runs. We can even factor in things like getting hit by a pitch (0.722 runs) and stealing a base (0.200 runs) or getting caught trying to steal (-0.377). Taking these components (note: baserunning components are typically excluded) we can get a wOBA for an individual player. Let’s look at Steve Pearce last season to make this tangible.
Our formula is:
wOBA = (0.689*uBB + 0.722*HBP + 0.892*1B + 1.283*2B + 1.635*3B +
2.135*HR) / (AB + BB – IBB + SF + HBP)
Last season Pearce had 40 unintentional BBs, 4 HBPs, 52 1Bs, 26 2Bs, 0 3Bs, and 21 HRs. He also had 1 intentional BB, 1 sac fly and a total of 338 at-bats. Plugging those numbers into our equation we get:
wOBA = (0.689*(40) + 0.722*(4) + 0.892*(52) + 1.283*(26) + 1.635*(0) +
2.135*(21)) / ((338) + (40) – (1) + (1) + (4))
If you multiply it out you get:
wOBA = 155.025 / 382 = .404765
We can truncate the last few decimals to get .404 and then go to Fangraphs and check our work… According to FG, Pearce posted a .404 wOBA last season, so congratulations! We just calculated wOBA!
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So why use wOBA? Well it’s scaled to on-base percentage meaning you should be able to intuitively look at it and judge the overall offensive performance of a player pretty easily. Last season the league average OBP was .310 meaning that a league average wOBA was the same. It also more accurately assesses how many runs a player is creating with their bat.
I know that wOBA isn’t a super common stat, so we might all need some context to better understand what wOBA should look like for a typical player. To do that, we can consult Fangraphs who provides excellent context for the stat. The first table below gives a rough breakout showing how to interpret wOBA. Pearce’s 2014 season falls into the excellent category. It’s worth noting that league-wide wOBA was down (.310 last season vs. the .320 in the table) so Pearce’s .404 figure is even more impressive.
This next table provides average wOBA by position from last season (note: this was through the All-Star break). You can see that the traditional bat-first positions of 1B, DH, and the corner OF slots are the strongest positions offensively.
Finally we have wOBA over time. This helps show the decline in offense over recent seasons as the overall league-wide output declined each season from 2008/2009 through 2014. This shouldn’t come as a surprise if you’ve been paying attention to the scoring environment around baseball, but it’s interesting to see nonetheless.
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So now, the moment we’ve all been waiting for. Below is a table showcasing players that had at least 50 plate appearances as an Oriole last season, as well as a few other guys who might be big contributors in 2015. We can compare this table to the one above to get a decent idea of how well the O’s hitters compare to their positional peers and the league as a whole.
*Only shows wOBA for production while wearing an O’s uniform. †Did not play for Orioles in 2014, but their performance is relevant for the 2015 season.
Steve Pearce’s season really sticks out from his peers, but the most impressive name on that list is Alejandro De Aza who raked in his time as an Oriole. All in all, the O’s will have nine players for the 2015 season who posted above average wOBAs during their 2014 season (Markakis also posted an above average wOBA, but is excluded from the list of 2015 contributors for obvious reasons). Other players like Chris Davis and JJ Hardy have at least decent shots at climbing back over that league average mark.
The O’s are a strong team offensively, and wOBA bears that out. As a club their .323 wOBA in 2014 was 6th best in baseball. It’s easy to see why when you look at that list above. For a few years now the offense has carried this club, and wOBA gives us a different lens to look at that through. It’s really not anything that earth-shattering, it’s just a more accurate method of giving hitters accurate credit for things they’re doing on the field.
Special thanks to the FanGraphs glossary for assistance in creating this post.
Jeff was the owner of the Orioles blog Warehouse Worthy, which focused on making advanced statistics a part of the conversation for the average fan. Outside of baseball, Jeff is a graduate of Loyola University where he received his Bachelor’s and Master’s in Business Administration. The Maryland native currently works for an Advertising Agency in downtown Baltimore. Previously a contributor to Beyond the Boxscore, he joined Baseball Prospectus in September 2014. You can reach him at [email protected]