Was Michael Bourn's stolen base decline caused by changing leagues?

USA TODAY Sports

Michael Bourn saw his stolen base total and success rate fall sharply in 2013, his first season in the American League. Was the change in leagues to blame for his decline?


Michael Bourn led the National League in stolen bases in 2009, 2010, and 2011, then stole another 42 bags in 2012 before signing with the Indians as a free agent last offseason. In 2013 he stole only 23 bases, and also had the worst success rate of his career, at just 65.7%. Both during and after the season, there was a fair amount of speculation that the decline was due in large part to him having to adjust to a new league, with new pitchers and catchers to read, etc. Such an explanation sounds reasonable, but I had my doubts about it. I never found the time to look into it much, but now I have.

I went back 30 years (an admittedly arbitrary round number) and found every instance in which a player stole 30+ bases in a season, all while playing in one league, then spent all of the following season in the other league. For the 30 years I studied (1983-2012), there happened to be 30 such instances.

I wanted to see what happened to their base stealing in those instances, but using a raw total wouldn't be the most effective way of doing that, since changes in playing time would cause large swings. Instead, I calculated "stolen bases per 500 plate appearances" for each player/season. I realize this is an imperfect measuring stick, but I don't know of any feasible way to create the perfect measuring stick, and I think SB/500 PA is a reasonable one.

Bourn's SB/500 PA went from 29.9 in 2012 to 20.0 in 2013, a change of -9.9.

Is that drop fairly typical for base stealers changing leagues? That's what I wanted to find out.

The table below shows the players, along with their SB/500 PA for Season A (before the league change) and Season B (after the league change), and the change between those figures.

Player Years Ages Season A Season B SB/500 PA Change
Otis Nixon 1998-99 39-40 37.0 73.9 36.9
Vince Coleman 1995-96 33-34 42.0 63.8 21.8
Brian Hunter 1996-97 25-26 31.7 50.1 18.4
Juan Pierre 2009-10 31-32 35.3 46.3 11.0
Alfonso Soriano 2005-06 29-30 22.0 28.2 6.2
Brett Butler 1987-88 30-31 26.7 31.7 5.0
Brett Butler 1983-84 26-27 31.8 36.7 4.9
Ivan Calderon 1990-91 28-29 24.0 28.9 4.9
Vince Coleman 1993-94 31-32 47.6 52.4 4.8
Lance Johnson 1995-96 31-32 31.0 34.5 3.5
Scott Podsednik 2004-05 28-29 49.1 51.9 2.8
Tom Goodwin 1999-00 30-31 42.9 45.4 2.5
Otis Nixon 1993-94 34-35 44.2 45.6 1.4
Steve Sax 1988-89 28-29 30.6 30.0 -0.6
Raul Mondesi 1999-00 28-29 26.5 25.8 -0.7
Alex Cole 1993-94 27-28 37.6 36.4 -1.2
Rickey Henderson 1995-96 36-37 32.9 30.7 -2.2
John Cangelosi 1986-87 23-24 47.6 45.1 -2.5
Darren Lewis 1995-96 27-28 30.4 25.9 -4.5
Tim Raines 1990-91 30-31 45.5 36.0 -9.5
Roberto Alomar 2001-02 33-34 22.2 12.2 -10.0
Kenny Lofton 2006-07 39-40 30.7 20.6 -10.1
Mike Cameron 1999-00 26-27 29.9 18.7 -11.2
Stan Javier 1995-96 31-32 35.7 22.9 -12.8
Kenny Lofton 2003-04 36-37 24.6 11.2 -13.4
Rickey Henderson 1998-99 39-40 49.3 35.2 -14.1
Corey Patterson 2007-08 27-28 36.8 17.9 -18.9
Phil Bradley 1987-88 28-29 28.5 8.5 -20.0
Roger Cedeno 2001-02 26-27 48.1 22.2 -25.9
Kenny Lofton 1996-97 29-30 51.0 23.9 -27.1

If anything, the cutoffs I chose are skewed to make it seem that players do fall off when changing leagues, because I specifically targeted players with 30+ steals before changing leagues. If you expand the study to include players who stole 30+ bases after changing leagues, you find 10 more cases:

Player Years Ages Season A Season B SB/500 PA Change
Corey Patterson 2005-06 25-26 15.5 45.1 29.6
Juan Pierre 2011-12 33-34 19.0 42.1 23.1
Roberto Alomar 1990-91 22-23 18.6 36.9 18.3
Gary Redus 1986-87 29-30 31.9 46.9 15.0
Kenny Lofton 1997-98 30-31 23.9 38.7 14.8
Roger Cedeno 2000-01 25-26 41.0 48.1 7.1
Steve Finley 1990-91 25-26 21.4 25.9 4.5
Felix Jose 1992-93 27-28 25.4 28.8 3.4
Kirk Gibson 1987-88 30-31 22.9 24.5 1.6
Mike Cameron 1998-99 25-26 30.5 29.9 -0.6

Here are the players in the last 30 years who had a larger decline in SB/500 PA upon changing leagues than Michael Bourn's 9.9: Kenny Lofton, Roger Cedeno, Phil Bradley, Corey Patterson, Rickey Henderson, Stan Javier, Mike Cameron, and Roberto Alomar.

In addition to the instance in which they suffered a big decline upon changing leagues, 6 of those 8 players had an instance in which they changed leagues and their rate basically stayed the same (Henderson and Cameron) or went up (Lofton, Cedeno, Patterson, Alomar). So while in theory there could be certain players who struggle with changing leagues, that does not seem to be the case, because the players who struggled almost all have instances in which they changed leagues without suffering a big decline.

Furthermore, if players' base stealing falls off upon changing leagues because they have to adjust to new pitchers and catchers, the biggest declines should come from players moving to a league they've never been in before, but among the 8 players with the largest declines, 7 of them (all but Bradley) had a big decline after a league change that involved returning to a league they'd already played in (meaning it was a change, but not to a new league).

What happens following a big decline?

While I no longer think this data really matters, because I believe the declines have been shown to be part of a random distribution, not evidence that changing leagues impacts base stealing, I know some readers will wonder, and I have the additional data, so I might as well include it. Here are the 10 largest declines, along with their SB/500 PA for the following year (Season C):

Player Years Ages Season A Season B Season C
Roberto Alomar 2001-03 33-35 22.2 12.2 10.0
Kenny Lofton 2006-08 39-41 30.7 20.6 DNP
Mike Cameron 1999-01 26-28 29.9 18.7 26.9
Stan Javier 1995-97 31-33 35.7 22.9 24.5
Kenny Lofton 2003-05 36-38 24.6 11.2 27.1
Rickey Henderson 1998-00 39-41 49.3 35.2 34.7
Corey Patterson 2007-09 27-29 36.8 17.9 DNP*
Phil Bradley 1987-89 28-30 28.5 8.5 15.9
Roger Cedeno 2001-03 26-28 48.1 22.2 13.3
Kenny Lofton 1996-98 29-31 51.0 23.9 45.2

*Patterson did technically play, but only had 30 plate appearances all season, making his data fairly useless. He stole 2 bases, giving him a rate of 33.3, if you must know.

So, 5 of the 10 saw a rebound, 3 saw a decline, and 2 didn't really play. It should also be pointed out that of the 10, only Cameron, Javier, Patterson, and Cedeno actually spent all of Season C in the same league as they spent Season B, the others (including all 3 of Lofton's cases) involved the player returning to his league from Season A.

What happened in 2013?

As I said, when I first set out to look into the question of what happens to base stealers when they change leagues, I had it in mind to see how they did in Season C, and so I only looked for players who stole 30 bases before or after a league change through 2012, because those were the cases where a Season C already existed. It occurred to me that I may have missed a case by not including 2013. It turns out I was missing a bunch:

Player Years Ages Season A Season B SB/500 PA Change
Ben Revere 2012-13 24-25 36.2 32.7 -3.5
Shane Victorino 2012-13 31-32 29.3 19.7 -9.6
Michael Bourn 2012-13 29-30 29.9 20.0 -9.9
Drew Stubbs 2012-13 27-28 27.6 17.7 -9.9
Jose Reyes 2012-13 29-30 27.9 17.9 -10.0
B.J. Upton 2012-13 27-28 24.5 13.5 -11.0
Emilio Bonifacio 2012-13 27-28 54.8 30.4 -24.4

No other season in the study had more than 4 players (1996), compared to 7 for 2013, and all 7 of them fall into the "decline" side of things, 6 of them fairly substantially. I believe it's just coincidence that there were so many instances of a base stealer changing leagues, and that they all saw their stolen bases decline. I am open to suggestions if anyone believes it was not a coincidence, and has a working theory on why, after 30 years of random fluctuation, there's suddenly "consistent" decline.

Summary and Conclusions

* Between the 30 from the first chart, the 10 additional ones from the second chart, and these 7, there are a total of 47 cases from 1984-2013 in which a player changed leagues and stole 30+ bases before and/or after the switch.

* The changes in SB/500 PA among those 47 cases range from +36.9 to -27.1.

* 22 of the 47 cases had an increase in SB/500 PA, 25 had a decrease.

* The median change is -0.6

* The pool of players is not limited to young players, or to old ones, there's a wide range of ages (22 to 41) and the players are fairly even split between what might be considered young and old, as the median age is 30. Among 24 players 30 or older, the median change was +1.5. Among 23 players 29 or younger, the median was -0.7 (which seems to me like an insignificant difference).

* 6 of the 8 players who account for the 10 largest declines between in season from 1984 to 2012 also had another league change, in which their base stealing either increased or decreased only very slightly. Even when looking at individual players, changes in base stealing upon changing leagues seem to be random.

* 8 of the 13 SB/500 PA declines of 10+ came form players returning to a league they'd played in before, not playing in a league for the first time. If changing leagues hurts base stealing, it should hurt players new to a league more than players returning to a league, but the data does not support that at all.

Changing leagues has no real impact on players' base stealing, the changes you see are evenly distributed randomness, the same as I expect you would see if you looked at players who didn't change leagues. The slight decline is to be expected, because speed declines with age. Michael Bourn's decline is far more likely to be related to his moving out of his prime seasons than any other factor.

.

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