The Stolen Base Aging Curve Depicted
Over the last two years, my fellow FanGraphs authors have discussed the effects of aging on player performance and shared various pretty curves. Eno Sarris introduced some hitter aging curves for both batted ball type distribution and isolated power. Then Bill Petti looked at the various plate discipline metrics. The one skill missing thus far has been speed.
There has yet to be a perfect metric we could look at to determine who is truly the fastest runner in baseball. There are many statistics we could reference that serve as a good proxy though, such as stolen base attempts, Spd score, triples totals, and infield and bunt hits. Up to this point, aging curves have focused on skills and ratios. But aside from Spd score, all the other indicators of speed are results oriented. While it is preferable to look at a more skill-driven metric, a stolen base aging curve still provides meaningful information.
Thanks to the mathematical wizardry of Jeff Zimmerman, presented below is the stolen base aging curve from 2002 to 2013 per 600 plate appearances.
The graph includes curves for two groups of players. The blue line with a flatter slope represents the entire population that was analyzed. The steeper orange line is composed of every player who stole at least 20 bases in any season during the time period examined.
The obvious explanation for why the All group line is flatter is because its stolen base total begins at a lower base. So the decline is going to be muted since it doesn’t have as far to fall. On the other hand, the average from the 20 steals group is higher, so when the speedsters start losing their ability to swipe bags, their stolen base total goes into free fall. While the slope is certainly meaningful, perhaps more important is the age of decline.
We have always known that speed is a skill of the young and the curves validate that this is indeed true. Whichever curve you choose to look at, a player’s stolen base peak comes during the same age range — 25 to 26. For those who were never true burners to begin with, the decline from there is gradual, eventually resulting in about a seven steal loss by age 38. For the speed demons, the slow down occurs much more quickly, with the same seven steal loss happening around age 30. By age 38, the group lost 16 steals off their peak.
So from a fantasy standpoint, the obvious conclusion is to target stolen base threats who will be between ages 25 and 26 in 2014 and perhaps reduce your stolen base expectations for players the further they are away from that sweet spot. The following fantasy relevant stolen base threats will fit into that age range next season:
Elvis Andrus
Starling Marte
Leonys Martin
Ben Revere
DJ LeMahieu
Paul Goldschmidt
A.J. Pollock
Josh Rutledge
Kyle Seager
Justin Upton
Domonic Brown
Of course, each player needs to be analyzed on his own merits, but just like the magical age 27 breakout theory group unveiled on fantasy sites each year, this could be your group of stolen base breakouts. Or at the very least in the case of the three names on top, they will be able to sustain their high totals of 2013.
Mike Podhorzer is the founder of ProjectingX IQ, an advanced fantasy baseball analytics platform that transforms projection data and in-season performance signals into actionable intelligence. He is the 2015 Fantasy Sports Writers Association Baseball Writer of the Year and three-time Tout Wars champion. He is the author of the eBook Projecting X 2.0: How to Forecast Baseball Player Performance, which teaches you how to project players yourself. Follow Mike on X@MikePodhorzer and contact him via email.
Very useful. Seems logical that the body’s aging and mounting injuries would prevent people from attempting to steal, even when success rates have been pretty good. In particular player cases, we of course have to factor in managers, too–which is why I feel particularly cursed by figuring I’d get 15+ steals from Heyward and J. Upton this year.
This might be a really stupid question, but why look at the 02-present period? Steals didn’t really take off until the 60’s, peaked in the 80’s (anecdotally), and have changed a lot in the last decade plus as teams became much more mindful of sb% and leverage in giving the greenlight.
Why not start in, say, 1960? That way, more of the great thieves (Brock, Henderson, Raines) would be included. Only Reyes (sorta) really measures up to them among active players and he’s past his baserunning prime.
Basically, players should be compared during similar ERA’s. Success rate is more important now to keeping the numbers up than in the past.
“There has yet to be a perfect metric we could look at to determine who is truly the fastest runner in baseball.”
Billy Hamilton. Next.
Fuck DAT shit.
So in other words, avoid Alex Rios. 🙂
Can we see a third line — that of players who had 2 or 3 15-steal seasons in that period? Seems like the possibility of somebody having one great steal season in their career might make the orange line fall more precipitously. I think the a question that should be examined is how do players who are acknowledged base stealers age? Using a player pool of individuals who had at least 2 or 3 seasons with 15 steals might provide some insight there. Thanks.
The curves are almost identical.
http://www.fangraphs.com/fantasy/wp-content/uploads/2013/12/SB2.jpg
It is not surprising with 159 in the >= 20 SB group and 136 in the 15 SB x 2 group.
I’d be really interested to see this same chart, with the Y-axis presented as percentages rather than raw stolen bases. In its current form, it’s really useful for evaluating the aging curve for mid-range players, but for outliers not so much. For example, players that have stolen 60+ bases at one point are probably going to see a reduction of more than 15-16 bases in their late 30s. If the data were expressed as percentages, I think you might end up with something more useful for all players (including those that never stole much to begin with).
Don’t mean to criticize too much, it’s great work!
(when I say “percentages”, I don’t mean stolen base success rate, I mean percentage decrease in overall stolen bases. i.e., if a player peaked at age 26 with 20 stolen bases, and then had 10 stolen bases at age 30, the percentage would be -50%.)
Just in case that wasn’t clear!
This. I raw losses are not very useful for this kind of aging curve, I think. It could be much more informative to see how the curve flexes with respect to the peak (or, in other words, charting the % increase/decrease we’d expect at each year).
I agree heartily with the last 2 comments. The number of fewer SB’s is not nearly as informative as the percentage drop.