Full disclosure: I'm not sure I have any real conclusions on this topic, I just had some analysis I wanted to share and virtually "think out loud."

Raw data from Baseball Savant:

Yearly Baserunning Value

Total Baserunning Value

Baserunning Value

The Brewers have built an impressive and speedy roster, led by Garrett Mitchell, Jackson Chourio, Blake Perkins, and Brice Turang -- all with a sprint speed of over 29 feet per second. That value is calculated by Baseball Savant as:

Quote

A measurement of a player's top running speed, expressed in "feet per second in a player's fastest one-second window.

29 feet per second would equate to an 11.3 second 100 meter time (not quite Olympic material), 19.8 mph, and a 4.14 40-yard dash (better than any recorded in NFL history). Of course, those times are not perfectly equal, since the difference between one and four seconds is an eternity in the world of sprinting, but it does put things in perspective. 

Along with raw sprint speed, Baseball Savant also calculates the run value for each base running opportunity:

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Based on inputs including runner speed, outfielder throwing arm, runner position on the base paths, and outfielder distance from both ball and bases, an estimated success probability can be created for each opportunity. With that information available for each play, the player’s actual success rate can be compared to the estimated rate and the cumulative metric can be created, accounting both for bases taken (prevented, for outfielders) and chances taken or not. This does account for extra bases taken by batters or runners on batted balls; it does not include stolen bases, as it’s about taking extra bases against fielders.

More on the stats at Tom Tango's site.

Christian Yelich

Christian Yelich is the Brewers' poster boy for creating runs on the base paths, with the 4th-most runner runs in MLB since 2016, behind Trea Turner, Kevin Kiermaier, and Billy Hamilton. He has done this, despite having a slower sprint speed than many others who have provided similar amounts of damage on the bases. Clearly, he is a smart and efficient runner.

But much of the running value that he has provided is due to the fact that he is such a good hitter. He has a lot more opportunities to add value because he is on base more often than many others with similar speed profiles. If we look at the running value added per opportunity, his standing slips a bit.

Since 2016, Yelich's sprint speed has ranged from 28.1 to 28.9 ft/sec, with a median of 28.4. If we look at the runners with a similar sprint speed, we find players like Brett Gardner and Ozzie Albies who have about the same sprint speed but are more aggressive than Yelich. Albies is an interesting comparison -- he has 75% fewer at-bats than Yelich, but is worth just 87% fewer baserunning runs. The difference seems to be in the value under runs lost on the basepath due to holding. That is, Yelich will not risk taking the extra base often enough. When compared to all other runners near his same speed over the past 8 years, Yelich has an average attempt rate. He attempts taking the extra base 40% of the time and runners with similar speed advance 39% of the time.

Baseball Savant will take you right to the video of the plays where Yelich should have gone for the extra base and there are a few similarities. The highest run value lost of his holds is only -0.24 and is a misplay by a deep second basemen. But Yelich wasn't running hard enough out of the box to take advantage of it. (Not sure how to or if I can embed all of these videos, so they're left as links).

https://baseballsavant.mlb.com/sporty-videos?playId=c4b987d2-c773-4fb6-854a-bced6446f78f

There are other plays similar to this:

https://baseballsavant.mlb.com/sporty-videos?playId=17811a50-aa96-401a-bc75-f3fa09a095f8

Looks like he is jogging out of the box, thinking it was a home run or an out.

https://baseballsavant.mlb.com/sporty-videos?playId=a0aa8650-6d9d-4ee1-af05-d5c489da2cd9

Up by 8 in the game, so might have not been as aggressive, but didn't read the line drive properly to fall in.

https://baseballsavant.mlb.com/sporty-videos?playId=8d4f2379-f72f-453c-9a4a-35a854f76046

Didn't read the fly ball well enough to know that it would fall.

https://baseballsavant.mlb.com/sporty-videos?playId=5c068b59-4f5b-47a2-9a51-3753b60a7ff5

Didn't read the fly ball to score on one over the fielder's head.

So it appears that Yelich will sometimes misjudge where the ball is hit, either on his own or other people's fly balls.  There aren't a lot of egregious errors, but just so many of them that they add up.

Brewers

But I also wondered if the reduced aggression on the bases is something the team implements. It's hard to say that since they are currently 2nd in steals in all of MLB, but that is to be expected because of their team speed.

Baseball Savant gives a statistic that shows how often the baserunner attempted to advance in comparison to the average runner. But that doesn't tell the whole story, since a faster baserunner is more likely to advance. So how do the Brewers compare to other runners with similar speed in the same situation?

It does appear that all of the Brewers speedy runners are less aggressive than their counterparts when it comes to taking the extra base. Everyone, except for Blake Perkins. And his smart baserunning has led to an impressive amount of value on the bases for how few opportunities he has had. He would be worth nearly one run more if he hadn't run into an absolute perfect throw from Fernando Tatis Jr.

https://baseballsavant.mlb.com/sporty-videos?playId=e99b100b-0f86-4232-a3cb-69a0cfcb8e8b

Perhaps Jackson Chourio, Brice Turang, Sal Frelick, and even Christian Yelich could tweak their aggressiveness up 3-5% and add even more value stealing another 90 feet?

But how is their aggression or passivity affecting them in terms of runs on the basepaths?

Well that's where it's interesting and I'm not entirely sure how to parse this into a conclusion. If we take their running value and divide by opportunities to advance, we see some interesting trends. Blake Perkins is clearly very aggressive, but only average in comparison to runs per opportunity, though that is skewed by one good play from Tatis. But Jackson Chourio is very passive and also average in runs per opportunity. Brice Turang and Joey Ortiz are substantially below league average for their speed in terms of runs of advancing on the bases for the chances they get.

But if we look at their runs of advancement when they try to go, now they are a little closer to league average at their speed. Both Turang and Ortiz come a bit closer to the average.

So perhaps Ortiz, Turang, and Chourio can gain some lessons from Yelich on how to read balls a little better and take an extra base here and there. Perhaps Hoskins, Bauers, and Contreras can be a little more aggressive. The Brewers are currently ranked 8th in MLB for baserunning runs, oddly enough much of those coming from being too aggressive. They have four plays at the plate where they were thrown out, along with five more plays that lost at least -0.74 estimated runs. It appears that they could be more aggressive based on their speed, but have lost more runs than nearly any other team because of being too aggressive.

If nothing else, it is an interesting jumping off point to discuss how the team has approached baserunning this year.