Posted by Doug on September 28, 2006
Shaun Alexander's injury prompted a thread over at the footballguys messageboard about running back injury rates. This article, which I wrote over five years ago, was referenced.
Whenever something I wrote over five years ago is referenced, I wince. Why? Because I was an idiot five years ago, that's why. It's always embarrassing to realize how much of an idiot you were five years ago, but I have convinced myself that it's better than not thinking you were an idiot five years ago. With rare exceptions, if you look at something you wrote five years ago and do not now see lots and lots of ways to improve it, that's a bad thing. It means your writing is not improving. That's what I tell myself, anyway.
In that old article, I looked at all running backs who played in a certain number of games in Year N, and then measured how many games they played in Year N+1. In order to be able to plausibly assume that all games not played were due to injury, I only included players who met a certain performance benchmark in Year N. That benchmark was 6 fantasy points per game, which I now realize was too low. It's quite possible that some of the games missed in Year N+1 were due to benchings rather than injury.
So I re-ran the study with a more relevant data set. I looked at all top 10 (in terms of fantasy points) running backs who played 16 games in Year N, with Year N ranging from 1988 to 2004. It would be pretty rare for a running back to be flat-out benched so quickly after a top 10 year, so it's relatively safe to assume that any missed games the following year were due to injury. There may be a very rare benching or a suspension or two in the data, but I doubt it is messing with the numbers too much. Since I required 16 games in Year N, we are looking at backs who presumably had no pre-existing conditions entering the season. So the question we are answering here is: if a running back was productive last year and healthy entering training camp, how many games do we expect him to miss due to injury this year?
Answer: 2.4 games. Here is the distribution:
That means that 48.8% of the previously-healthy-and-productive runners played 16 games the next year, 15.0% played 15, and so on. The interesting thing is to compare this distribution to that of the running backs who were in the fantasy top 10 and did not play 16 games in Year N.
That's an average of about 2 missed games, and a slightly greater percentage of backs playing the full 16. So what we're seeing here is a lack of evidence that past injury history is an indicator of future injuries.
Now, there were 127 backs in the first group and only 40 in the second group, so we're into small sample size territory. Let's move the cutoff to top 20 instead of top 10. This is the next-year's-games distribution for all fantasy top 20 running backs (1988--2004) who played less than 16 games:
That's an average of 3 missed games on the dot. So maybe previously-injured running backs do have a higher injury rate, but maybe not. And even if so, it doesn't appear to be much higher.