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Royals base running aggression in 2023

The Royals took advantage of the new rules to steal more bases than most teams, albeit at a slightly worse success rate.

MLB: New York Yankees at Kansas City Royals Jay Biggerstaff-USA TODAY Sports

The rules changes were a huge part of baseball’s story last year, so as we head into another year under the new rules I wanted to review what the Royals did or did not do to take advantage of them. First up, base stealing.

Under the new rules there were two changes that improved the odds of swiping a bag. One, the bases themselves were made bigger, which makes the distance between them just a little bit shorter for the runner. Two, pick-off attempts were limited. If a pitcher threw over to a base/disengaged three times, it was a balk. The Royals, being a young team with a lot of good runners, was well positioned to take advantage of this. Almost everyone on the team was at or above league-average in sprint speed with the only exceptions being Salvy, Vinnie, and Matt Duffy. Sprint speed is not all you need to steal bags, but it certainly helps.

At the beginning of the season, the Royals were stealing at about the same rate as the league, but in June the kicked it up a notch and stayed above league-average the rest of the way.

They went from an attempt every 40 to 50 plate appearances to almost double that, while the league rate was consistent across the season.

Stealing more often is only good if the success rate doesn’t crater, however. The more you run, the lower the success rate will become unless you are way below the optimal attempt rate. The Royals did see their success rates dip a lot in June, but it rebounded some and was pretty solid the rest of the season.

Their success rates were above the league-average when attempting steals at league-average rates, with below-average success rate when their stealing became more aggressive. The league ended up with an 80.2% success rate on the year, and the Royals ended at 77.6% after being at 75.5% from June through September. Kansas City’s June looks a little out of line from the rest, so if we want to throw it out they were 77.2% in the July through September time frame.

In general, you want to stay above 70% in stolen base success rate, though it is more complicated than that when you break things down situationally. Overall, I would say that the Royals managed a solid enough rate to call this a successful level of aggression. If you calculate their theoretical steals at their success rate and league attempt rate, then you see they stole 53.36 extra bases and got caught 15.39 more times than they otherwise would have in expectation.

In general, the true value of this is pretty small, but the aggression paid off in the aggregate. If they had run like they did in April and May with the higher success rate, then the difference from being more aggressive was probably only about two runs total relative to what they might have been able to do. That is assuming they could have maintained an 84% success rate, which is not guaranteed. There can be other benefits to running a lot outside of run expectancy charts. Just putting pressure on the pitcher is likely worth something.

There you have it, the Royals had one aspect of their team outperform the league in a terrible season. Unfortunately, it is one of the least important areas when it comes to correlation with team success.