wOBA: Weighed On-Base Average

We fans want a number to measure and compare the quality of our baseball players. Trouble is, so many of the numbers we’re comfortable with don’t really give us a full picture.

Batting average (abbreviated as BA or AVG), though traditional and ubiquitous, is perhaps the most flawed of all. On-base percentage (OBP) is more useful, as is slugging percentage (SLG), though the attempt to give us a holistic look by combining those two into one measure, OPS (literally “on-base percentage plus slugging”), doesn’t really work for a number of good mathematical reasons.

We rely on slash lines of AVG, OBP, SLG, despite their one dimensionality, just as we take our temperature to see if we are running a fever, knowing full well that the absence of fever does not necessarily mean we’re healthy.

wOBA: A Better Performance Metric

For evaluating both batters and pitchers, Keith Law, in his excellent book Smart Baseball, recommends weighted on-base average (wOBA), a sabermetric based on linear weights that was introduced in the 2006 book The Book: Playing the Percentages in Baseball by Tom Tango, Mitchel Lichtman and Andrew Dolphin.

wOBA covers all the typical batter’s events (hits, extra base hits, walks, times hit by a pitch, and outs) and applies weights to each of those components that correspond to their impact on the game.

wOBA weights aren’t applied as simplistically as they are with SLG, which wrongly assumes that triples are worth three times more than singles, for example. Instead, weighting for wOBA is systematically determined based on the run scoring environment and updated every year. As baseball changes due to any number of external events, the weighting adjustments used in wOBA make these individual feats more valuable and, naturally, the converse is true. When, for example home runs, are more plentiful, the weighting reduces their value in the wOBA calculation.

Weighting values used in determining wOBA can be different depending on who is doing the calculating. RSNStats relies upon the weights determined by the Fangraphs web site, which makes them available for all years going back to 1871.

In 2020, the formula for wOBA was (0.699×Unintentional BB + 0.728×HBP + 0.883×singles + 1.238×doubles + 1.558×triples + 1.979×HR) / (AB + BB – Intentional BB + SF + HBP).

Understanding Expected wOBA

As with other advanced stats, there are so-called “expected” versions. You’ll recognize these because they start with the letter “x.”

In the case of wOBA, the expected version, xwOBA, can give a clearer view of a batter’s skill because, as former MLB data scientist Sam Sharpe writes, xwOBA focuses on certain “tracked skills to reach a conclusion about what would have happened to balls in play under completely average MLB game conditions.” By eliminating the effects of defense on an offensive effort, expected stats like xwOBA rely on determining a batter’s quality of contact using factors such as exit velocity and launch angle of the batted ball and the batter’s sprint speed to determine the expected outcome as opposed to the actual outcome.

You may be tempted to say you only care about a player’s actual performance and not what typically occurs. But since batters can’t control what happens when the ball leaves the bat, it’s useful to evaluate the player’s contribution based on what typically happens given a certain combination of offensive characteristics.

For example, a batter who reaches safely on a misplayed ball gets lucky. wOBA credits the player with the result. But xwOBA likely concludes that the batter’s performance, as measured by a variety of characteristics, would under average conditions, likely been an out. xwOBA, therefore, may give you a more realistic understanding of the player’s actual skills.

More Ways to Evaluate Pitchers

Another useful metric for evaluating pitchers is FIP: Fielding Independent Pitching. This is because once a ball is put into play, there’s not much difference in talent among pitchers. As Tom Tango notes, what you see might tell you there’s a difference but much of that is random variation.

Pitching wins are really immaterial. More reliable measures for evaluating pitchers are their strikeouts, walks, hit-by-pitch and home runs allowed. These are the actions that a pitcher truly controls.

FIP abstracts away the fielding plays to give you a measure, much like Earned Run Average (ERA), of the runs allowed by what the pitcher controls most.

Learn More

There’s little reason to create a new glossary for stats here at RSNStats.com. Rather, if you’re interested to know more, check out the recommended books (above) or the explanations available on the Fangraphs, Hardball Times, and Major League Baseball web sites.