Introduce ISO

Last time, I looked at how well a basic calculation like Speed Unit can compare to an in-depth statistic like Ultimate Base Running. It turns out that it does pretty well. How about bat speed and exit velocity? I have been told that Isolated Slugging is a is a crude measure of ball speed, but that was before Statcast was around. Isolated slugging is calculated as (2B + 2x3B + 3xHR)/AB, or SLG - AVG.

Is that true? How about some other hitting statistics? How well can we estimate a hitter's bat speed or exit velocity based solely on basic hitting stats? This becomes an important exercise when we want to look at data during times and places when bat speed and exit velocity are not available.

Baseball Savant makes it pretty easy to compare the yearly values of many of these statistics. I encourage you to follow this link to an interactive scatter plot and check out some of the correlations yourself.

ISO and Exit Velocity

Here's the first one I was most interested in. These are all individual player seasons and we can see a 0.36 correlation coefficient for the 1392 qualified player seasons since 2015. That means ISO accounts for just 36% of the variation in average exit velocity for a batter -- a crude measure indeed. But better than nothing, and there is a clear trend.

But we can also look at the career values for the players during the Statcast era as well.

That is actually a much stronger correlation. Clearly there is some seasonal variability, even in average exit velocity for batters. There are some speedsters down on the bottom left (Billy Hamilton, Dee Strange-Gordon, Billy Burns, and Victor Robles) and some of the game's top hitters in the top right (Shohei Ohtani, Aaron Judge, David Ortiz, and Mike Trout).

Oneil Cruz stands out as having a lower ISO than expected based on his average exit velocity, and I'm not really sure what is happening with him. It doesn't appear that he is hitting a lot of ground balls, so perhaps he is just hitting the ball hard right at the fielders. I'll leave that as an exercise for the curious Pirates fan.

William Contreras is the Brewer the farthest away from the average line. His ISO of 0.159 would predict an exit velocity of 88.5, but his actual exit velocity is 91.7. We might expect his slugging to increase a bit based on this.

Barrels

But exit velocity is not the end-all, be all. Some players track their "Barrels." Those are batted balls with the perfect combination of exit velocity and launch angle.

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To be Barreled, a batted ball requires an exit velocity of at least 98 mph. At that speed, balls struck with a launch angle between 26-30 degrees always garner Barreled classification. For every mph over 98, the range of launch angles expands.

This is Aaron Judge's realm, where he leads all players by nearly 5% over the last 9 years.

This is actually an even better correlation. It makes sense--the better you strike the ball, the more likely you are to hit doubles, triples, and home runs.

Bat Speed

How does this compare to the newer statistic that we have in 2024 of bat speed? Exit velocity and bat speed are correlated to each other, but not perfectly.

For instance, Christopher Morel is 6th in qualified hitters in bat speed this year at 76.3 mph, but his average exit velocity is 69th at 89.6. For the Brewers, Joey Ortiz stick out as someone whose average exit velocity would be expected to be higher based on his bat speed. His 74.4 mph, 46th-ranked bat speed doesn't align with his 204th ranked exit velocity of 87.9 mph. Despite swinging 8 mph harder than Turang, he produces almost the same exit velo.

So far, ISO doesn't correlate very strongly to pure Bat Speed, but we do only have part of a year's worth of data. We've already seen the increase in quality from a full year to multiple year's worth of ISO and exit velocity.

But ISO, Exit Velocity, Barrel%, and Bat Speed are not the only things we can compare. Bat Speed has even less of a correlation with wOBA than ISO, at 0.12 so far for qualified batters. Luis Arraez and Steven Kwan prove that a batter doesn't have to swing hard to be productive, and Brice Turang is looking to build his own career using a similar strategy.

Historical Brewers

I'll close this like the previous article, by looking at some of the top historical Brewers seasons for isolated slugging. These are players with the highest consistent exit velocities and highest barrel rates. But these values are not simply taken as the raw isolated slugging percentage. These values are calculated as a z-score, using the average and standard deviation of the league's ISO that year.

Ryan Braun shows up twice, in 2007 and 2012, but not in his 2011 MVP year. Christian Yelich appears in 2019, but not his MVP season of 2018 (though he should have won that year anyway).

Onto the minor league leaders:

A rookie Rock shows up again, as he did for the abbreviated 1970's Speed Unit table.

Finally, the top 5 per decade for both the Major and Minor Leagues: