The Statcast system – Doppler radar and high-definition optical cameras – is installed in every major league park. Since 2015, Major League Baseball has tracked movement in a baseball game at 30 frames per second. On any pitch, Statcast records more than 600 measurements, which over a season is four billion data points.
This deluge of information provides more ways to measure and evaluate players. Yet, critics say it detracts from the enjoyment of baseball. But for the average fan, data can produce richer narratives. Did Scott Schebler’s shoulder injury lower the velocity of his throws from the outfield? Is Joey Votto swinging at a steeper launch angle? Is Raisel Iglesias throwing his slider with more break? Is Billy Hamilton the fastest runner in baseball?
For the business of running a major league team, wonderment takes a back seat to analysis. Measurement and evaluation are crucial to understanding your team and players you are interested in acquiring. The big payoff is if an analytics department can design leading-edge statistics using that data to better predict player performance and therefore uncover meaningful markers for building rosters.
A few stats based on the new data are making their way into the public sphere. One is expected, weighted on-base average (xwOBA), which is a way to evaluate hitters.
Can xwOBA offer insight into the Reds roster and pending offseason decisions? Before we get to that, let’s work through what the statistic is and then what it means.
[If learning details of new statistics doesn’t crank your gears and methodology makes your eyes glaze over, skip to the final section titled How xwOBA Informs Reds Decisions for the fun stuff.]
Start with On-Base Average (OBA)
The starting point for xwOBA is an older statistic, on-base average (OBA). OBA is another name for on-base percentage (OBP). It’s an easy statistic no matter what you call it. OBA measures how often a hitter gets on base. More formally, OBA is a rate statistic that credits a hitter with each safe event (walk, hit-by-pitch, single, double, etc.).
OBA does share a deficiency with batting average (AVG); it doesn’t differentiate between positive outcomes. All ways of getting on base count the same. Home runs improve your batting average and OBA the same as singles. So as an individual measure, it offers no way to show how much power the hitter possesses.
One way to fix this shortcoming is to count the extra bases a hitter achieves – one extra base for a double, two for a triple and three for a home run. This is the basis for the popular statistic we know as Slugging Percentage (SLG) and a more modern formulation, Isolated Power (ISO).
While SLG and ISO offer additional important information to AVG and OBP, they aren’t as precise as possible. Doubles aren’t quite worth twice singles in creating runs, home runs aren’t four times better.
To address that we can weight positive hitting outcomes based on run creation instead of extra bases. Every method of getting on base contributes to run scoring in one or more of three ways: (1) driving in runners already on base, (2) advancing runners already on base, and (3) putting a new runner on base. Each type of safe event affects those three factors differently.
That brings us to “weighting” OBA based on run creation.
Weight Hitting Outcomes
Weighted on-base average (wOBA) is a statistic first introduced by Tom Tango, Mitchel Lichtman and Andrew Dolphin in The Book: Playing the Percentages in Baseball (2006).
wOBA weights each positive hitting outcome (HBP, unintentional BB, 1B, 2B, 3B, HR) by the average number of runs it produces. The weights aren’t hypothetical. For wOBA, run production is based on actual baseball games played across the league in a given season. For example, the weighted value of a single comes from the average run production of every single hit in major league games. Assigned weights aren’t theoretical; they are empirical.
[The reason you haven’t seen wOBA cited at Redleg Nation much is that it’s basically the same as weighted runs created plus (wRC+) which we do use frequently. wRC+ takes the data for wOBA and puts it on a 100-point scale (that’s the “plus”). A 100-point scale makes comparison to average and calibrating year-to-year variations easier.]
Back to weighting on-base outcomes. Here are the weights for each safe event so far in 2017:
-
wBB = .693
-
wHBP = .723
-
w1B = .877
-
w2B = 1.231
-
w3B = 1.550
-
wHR =1.976
The weights are constructed to produce a league-wide wOBA equal to league OBA.
Weighting transforms a statistic (OBA) from one that doesn’t differentiate between types of hits to one that reflects the productiveness of the hitter in terms of run creation. And not just his runs batted in (RBI), which is easy to count; also the precise role that advancing runners and putting runners on base plays in scoring runs.
wOBA had been around for a decade before the Statcast data arrived. That brings us to expected weighted OBA.
Add Expectations
Statcast measures batted balls for Exit Velocity and Launch Angle. Every combination of Exit Velocity and Launch Angle produces a specific average wOBA. Again, these aren’t hypothetical numbers. The wOBA values are based on the outcomes of baseballs hit that way this season. For example, take every ball hit at 103 mph Exit Velocity and 25º of Launch Angle and figure out how many runs they produced.
From that set of data – the wOBA for every EV and LA combination – we can figure out how many singles, doubles, triples and home runs a batter should have hit, independent of fielding/defense and random chance about where the balls fall. That’s because we know the EV and LA combination on every ball that every batter did actually hit.
Take that data for a hitter’s actual batted balls, and add in established run created weights for his actual walks, HBP and sacrifice flies, and you come up with a statistic that shows what his run production was, independent of factors beyond the hitter’s control.
That’s expected, weighted OBA.
xwOBA removes random elements of defensive plays and luck. Like wOBA, it’s put on the scale of league OBP. League average OBP is (.322).
What is a good wOBA or xwOBA? Here is a guide:
-
.390 = Top 10 of qualified players
-
.370 = Great
-
.340 = Above Average
-
.322 = Average for qualified players
-
.310 = Below Average
-
.300 = Poor
-
.290 = Bottom 5% of qualified players
Research shows xwOBA offers better predictive value of a hitter’s future run production than his actual wOBA.
This is a fancy way of backing up traditional baseball scouting. For decades, baseball scouts have said something like: “Player A is only batting .250, but he’s sure been hitting the ball hard. The hits just haven’t fallen in. I expect he’ll produce more in the future.”
Today, radar and optical cameras allow us to take that scout’s insight, validate it and attach an exact number. That’s a powerful tool.
The top 12 major league hitters by xwOBA include: Aaron Judge, Mike Trout, J.D. Martinez, Freddie Freeman, Paul Goldschmidt, Anthony Rizzo, Giancarlo Stanton, Bryce Harper … and Joey Votto. So it’s a decent sorting stat.
OK, that’s the method. Let’s apply it to the Reds and a few of their upcoming roster decisions.
How xwOBA Informs Reds Decisions
Here’s a table showing the wOBA, xwOBA and the difference between the two numbers for the Reds regular position players.
The final column is the player’s xwOBA rank among hitters with at least 200 plate appearances. 320 players – or about 10 per major league team – meet that benchmark.
Positive numbers in the third column mean expected hitting exceeds actual hitting and argues for optimism regarding that player going forward. Negative numbers (in red) indicate the opposite. Remember, studies show that xwOBA predicts future run production better than wOBA.
Do these numbers tell us anything about decisions confronting the Reds this offseason?
Well the data is in a table, so it must mean something. Or four things.
1. Zack Cozart and Scooter Gennett
Bright red warning signs flashing for both players. Their expected production based on how they’ve hit are well below actual production. This confirms a common sense reading of their career numbers. xwOBA is a strong indicator that neither player will repeat their huge 2017 seasons. In Cozart’s case, a xwOBA of .336 is still well above average.
2. Adam Duvall and Scott Schebler
Duvall and Schebler have had similar years in actual production (see their respective wOBA). Which one to keep? Other factors offer split advice: Duvall has an edge in defense, Schebler in age. But xwOBA strongly leans toward Schebler. He’s hit the ball hard and not seen the expected production yet. Duvall, just the opposite. Clear evidence for how the Reds should handle the crowded outfield.
3. Billy Hamilton and Jose Peraza
Hamilton and Peraza are at the bottom of many league-wide batting stats. That’s because neither hits the ball hard. So it isn’t surprising that xwOBA is awful for both, since it’s based on the quality of ball striking. But Peraza has a clear edge. xwOBA is another piece of evidence to throw on the pile that shows we shouldn’t expect Hamilton to get better at the plate.
4. Joey Votto
We aren’t worthy of Joey Votto. But we didn’t need Doppler radar pulses to tell us that.