by Zachary O. Binney
In 2015 we published an analysis that found the number of reported NFL injuries has been rising substantially since about 2007. The word "reported" is important in that sentence. Our database, as with any database reliant on public information, contains only injuries that appear on an injury report.
This means an observed increase could reflect a few different things:
- A true increase in the underlying NFL injury rate. That is, more players really are getting hurt.
- Reporting bias: better reporting of injuries that occur by teams. We might expect this to lead to more minor injuries being reported, but to have little impact on more severe injuries.
- Diagnostic bias: better identification (and subsequent reporting) of injuries that were previously occurring but had been unrecognized. We might expect to see this with concussions and other head injuries, especially.
To separate out these effects we make one large but defensible assumption: injuries that caused missed games have been consistently reported regardless of the year. There is a lot of room for teams to manipulate the injury report for minor injuries, but if you're out of the game, you're out of the game.
We took the injuries in our database and split them by severity (minor, no games missed; moderate, one or two games missed; and severe, three-plus games missed) and type (concussion/head injury vs. other injuries). If the increase is due primarily to diagnostic bias, we would expect to see large increases in concussions and much more moderate increases in other injuries. If the increase is due to reporting bias, we would expect to see large increases in the number of minor injuries reported, while more severe injuries remained relatively flat. If there is a true increase in the underlying NFL injury rate, we would expect all groups to exhibit an increase.
While in previous articles we have reported injury rates per 1,000 athlete-exposures (a player participating in one practice or game), we're going to just provide counts here instead. This is because during the regular season the number of athlete-exposures is expected to have remained roughly constant from 2007 to 2015; whenever someone gets hurt, someone else immediately takes their reps or roster spot. Only injuries occurring in the regular season are counted, because preseason roster sizes have changed over time.
Overall Changes in Injury Rate
This is an update of our previous analysis that includes the 2015 season; 2016 is not comparable with prior seasons because the "Probable" designation was eliminated from the game status portion of the injury reports. In Figure 1, we can see a pretty clear increase in the injury rate over time, particularly from 2007 it 2012 before it plateaus from 2012 to 2015. However, as noted above, this doesn't allow us to tease out a true increase from reporting and diagnostic bias.
Injury Rates Stratified by Severity and Type
Figure 2 depicts what we would expect if the increases are primarily, but not entirely, due to diagnostic and reporting bias rather than a true increase in the NFL injury rate.
Head Injuries: The number of head injuries increased regardless of severity. This is consistent with both better identification of head injuries and more careful treatment of those injuries that are identified. We could hypothesize that 1) better identification causes an overall increase in the number of concussions that are reported, and 2) more careful treatment causes increases in the higher severity groups since what would have been a "minor" concussion in earlier years becomes a "moderate" or "severe" concussion in later years.
Non-Head Injuries: The largest increase by far is for minor injuries, which increased steadily by about 50 percent from 2007 to 2015. Moderate and severe injuries rose steadily from 2007 to 2011 before dropping and then rising again, but the overall difference between 2007 and 2015 is about 10 percent. Taken together, these suggest that much of the observed increase is due to better reporting of minor injuries.
But how much is due to reporting and how much to true changes? To estimate that, we can do some back-of-the-envelope math. First, we'll assume (generously) that there has been a true increase of 10 percent in the NFL's underlying injury rate. Overall, there has been about a 34 percent increase in non-head injury rates, so 10/34 = about 30 percent could be due to a true increase and 70 percent due to better reporting.
The proportion of the increase due to better reporting is likely well above 70 percent for head injuries, but it's tricky to calculate without a group that has steady reporting over time.
A majority (at least 70 percent) of the increase in reported NFL injuries appears to be driven by increased identification and reporting of concussions and minor non-head injuries rather than an actual change in NFL injury rates. True injury rates likely increased about 10 percent from 2007 to 2015. That's not negligible, but it's much lower than the 35 percent or so that just looking at the overall change might suggest.