by Zach Binney
As those of you who have read my previous articles for FO know by now, my main beat is injuries. One related question that grabbed my interest recently is whether the apparent increase in injuries in recent years (which may itself be just an artifact of better data collection or more attention being paid to the issue of injuries) could be shortening NFL careers.
So, I started poking around to see if anyone had done an analysis like this. It turns out that in late February, the Wall Street Journal published an analysis of NFL career lengths with startling findings: the average career length of an NFL player had dropped from 4.99 to 2.66 years from 2008-2015. That's a drop of nearly 50 percent in just seven years, and most of the drop came from 2011 onwards! Making the findings even more interesting, the decline was as linear as it was precipitous, and it held across all positions. The article's graphic is reproduced below:
The article proposed several plausible theories for the decline. Injuries were a main one. It's also possible that teams are simply churning through more players than ever before (FO's data shows a gradual increase from 1,895 regular season players in 2007 to 1,962 in 2015, a 3.5 percent increase). The 2011 collective bargaining agreement (CBA) increased training camp rosters from 75 to 90 players and introduced a new more favorable rookie wage scale, so perhaps teams made a conscious decision to construct younger rosters and jettison older players more quickly.
When I saw these results, I was intrigued (as were others. How could I have not known about this? How could the NFL Players Association not be up in arms about their players collecting paychecks for only half the length of time they used to? I set out to replicate the analysis using the article's data source of Pro-Football-Reference (PFR). The short version: I couldn't. When I used PFR data, I found virtually no decline in NFL career lengths in recent years.
I reached out to the WSJ's sports section on Twitter to try and figure out what was happening. They quickly replied and did what the best scientists and analysts do -- they shared their raw data and walked me through exactly what they did.
It turns out they had used PFR's Football Encyclopedia of Players (the pages listing players by last name) to extract player names and the first and last years they played. Good idea in theory, but in reality this Encyclopedia includes any player who ever played a regular season NFL game and -- this is very, very important -- whatever other players PFR felt merited tracking. Even more importantly, the cutoff for these "other players" seems to have changed dramatically over time, becoming much looser in recent years.
In 2011 -- right around when the career length drop really starts taking hold -- PFR appears to have decided to start more broadly tracking players who never played in regular season games. What's more, the group of players PFR included broadened every year: for example, in PFR's Player Season Finder query tool there were 379 "0-game" seasons in 2011, versus 398, 485, and 533 in 2012, 2013, and 2014, respectively. There were only two such player-seasons total from 2000-2010. Of note, since I first pulled my data back in March 2016, PFR has (mostly) cleaned this tool of these 0-game seasons for 2011, 2013, and 2014, but the 398 from 2012 are still visible -- they appear to have some sort of "NULL" value for games rather than a zero and were not caught by whatever cleaning PFR implemented. All these 0-game players, however, remain in the Encyclopedia.
This meant that, in the WSJ analysis of PFR Encyclopedia data, 2011 and later years had lower-quality players, on average, than previous years, and that the problem grew over time. Lower-quality players are going to have shorter careers. The trend the article found isn't because NFL players, as a whole, are having drastically shorter careers. It isn't even primarily that there are more lower-level players being given a (brief) shot at the NFL, though that may be happening to some degree. It's because beginning in 2011 PFR decided to start tracking more players at the bottom end of rosters who never played.
I used a different method to mine only players who played in at least one regular season game -- a population with a simple, consistent definition that stays steady over time that is also more appropriate for assessing NFL career lengths (See the Appendix at the end of this article for details). Our own Scott Kacsmar touched on the issue of including players who never played when calculating career lengths in his series on NFL draft production. With due respect to our colleague Ben Muth, I'm not sure I'd want to count his one training camp with the Chargers in calculating the average career length of NFL players (though, to be fair, that's one more training camp than I'll ever have).
In this consistent and more appropriate population you don't see anything near the cratering of career lengths reported in the WSJ. You could argue that career lengths in 2012 and 2013 were at their lowest since 2005, about half to three-quarters of a season lower than the average from 2007 to 11. This might reflect the beginnings of a trend toward earlier retirements, but it's far from a rapid halving of career lengths. Looking at the chart more broadly, I'm inclined to think that the recent variation is just random and well within historical norms. The standard errors (basically, a metric for how precise our measurements are) bear this out, as well:
If you stratify by position, defensive linemen and running backs show larger recent declines (defensive linemen from 6.6 to 6.0 to 5.5 from 2011 to 2013; running backs from 6.5 to 4.5 to 4.6 over the same time), but no position shows the precipitous, sustained, linear decline shown in the WSJ article. In addition, retiring defensive backs and linebackers actually showed slight increases over this period.
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Now, career lengths are tremendously right-skewed, so before my statistics professors yell at me that the averages tell us NOTHING, LEBOWSKI, here are some side-by-side boxplots showing the spread of the data. As a quick boxplot tutorial for the unfamiliar: the bottom and top "whisker" ends are the minimum and maximum career lengths; the bottom and top of the box are the 25th and 75th percentiles; and the middle of the box is the median. This means that the orange box is the 25th to 50th percentile of retiring player career lengths, while the gray box is for the 50th to 75th percentile of that same value. The pattern looks about the same -- median career lengths haven't changed since ticking up from four to five years in 2006 with the exception of a blip to six years in 2011. The 75th percentile did drop from nine to eight years in 2012, and the 25th percentile from three to two years in 2013, but those years' retirements really don't look out of whack historically:
So what can we conclude? Players who play -- the main population that we're interested in when it comes to career lengths -- were playing about as long as they used to through 2013. This may have changed over the last couple of years, but PFR does not have the retirement data to let us investigate that. The original WSJ analysis was flawed because it included large numbers of players who never played from 2011 onwards but not before, making average career length appear to shrink when we really were just looking at lower-quality players.
One other big lesson: this is a cautionary tale. It's really, really, really easy to make mistakes like this, even when you get data from a reputable source like PFR and go through a robust series of error checks like the WSJ author did. I would just caution everyone to dig deep and make sure your data don't have any strange patterns (like a massive rise in 0-game players). Always question, and always look for yourself. This exercise was especially terrifying for me because I could see myself making exactly the same mistake.
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A couple last-minute notes: my numbers are higher than WSJ's across-the-board because, in addition to excluding players who never played, I defined career length inclusively (last year - first year + 1) instead of exclusively (last year - first year). These are in line with Scott's numbers in the draft series and those of the NFL from 2011. They are, of note, substantially higher than the roughly 3.5 years commonly quoted by the NFLPA, but that number is flawed because it reflects average experience at a cross-sectional point in time rather than the actual average length of a full career.
Also, I excluded 2014 because, in PFR data, a player's "last year" is listed as the last year in which they played a regular season game, even if they might still be active. So, for example, Jordy Nelson -- who tore his ACL and missed the entire 2015 season -- is right now listed as ending his career in 2014, which is likely false. This is likely to be a big problem for the 2014 season and a progressively smaller one as we go further back (since, for example, a still-active player would've had to miss the full 2014 and 2015 seasons due to injuries or other reasons to be erroneously listed as ending his career in 2013). I wouldn't be surprised if the modest downticks in 2012 and 2013 are partially driven by this or similar residual effects, though.
Appendix: PFR Query Methods for Players Who Have Played in One or More Regular Season Career Games
First, I extracted data on drafted players from 1980 to 2015 who played at least one game in the NFL using the Draft Finder Query. Second, I supplemented that with data on undrafted players over the same period playing at least one regular season game using the Player Season Finder query.
Zach is a freelance injury analyst and a Ph.D. student in Epidemiology focusing on predictive modeling. He consults for an NFL team and loves Minor League Baseball. He lives in Atlanta. You can contact him on Twitter @zbinney_nflinj.