Note: This is a two-page entry.
Every year during my fantasy drafts (I would say the exact number of leagues in which I participated last year if I wasn’t so embarrassed about the number–hint: I can’t even count them all with my fingers and toes), I hear a variety of fantasy football “truisms” thrown out following the selection of certain players.
“Wide receivers always break out in their third year.”
“Don’t draft a kicker until the last round.”
And perhaps most frequently, “Running backs are never the same the year following a season of 370 (or any other arbitrary number) touches.”
It is this last notion which will be the subject of this post. There have already been some informative studies produced on the decline of running backs following a season of heavy work, not the least interesting of which can be found here.
Before delving into the results, it is critical to once again rehash the importance of the correlation/causation distinction. In our article on the importance (or lack thereof) of offseason workouts, we wrote:
Both of these notions–running the ball and having a good coach–are onlycorrelated to winning. Correlation does not equate to causation. For example, intelligence is rather strongly correlated to shoe size. Does possessing big feet make you smarter? Of course not, but people with big feet are generally older, and older people tend to be more intelligent than children (although that is unfortunately not always the case).
Nonetheless, we only notice the presence of these characteristics when it is too late–they have no predictive power.
With the distinction between correlation and causation in the back of our minds, let’s examine the stats regarding a running back’s touches and his performance the following season.
Football Outsiders (a terrific site, by the way) completed a study on the workload of running backs and summed up their results as follows:
A running back with 370 or more carries during the regular season will usually suffer either a major injury or a loss of effectiveness the following year, unless he is named Eric Dickerson.
Terrell Davis, Jamal Anderson, and Edgerrin James all blew out their knees. Earl Campbell, Jamal Lewis, and Eddie George went from legendary powerhouses to plodding, replacement-level players. Shaun Alexander struggled with foot injuries, and Curtis Martin had to retire. This is what happens when a running back is overworked to the point of having at least 370 carries during the regular season.
Is 370 carries really a magical number by which we can judge the future effectiveness of a running back? It is certainly true that a running back coming off of a season with a heavy workload is more likely to be less effective and more likely to get injured than was the case in the prior season–but is this truly the result of the high number of touches, or is it due to something else?
The truth is that, while the statistics do point to a decrease in effectiveness and an increase in rate of injury following a heavy-workload season, these numbers are both insignificant and irrelevant.
The key is in a statistical term known as ‘regression toward the mean.’ Mathematics is a beautiful thing. Given a large enough sample size, numbers always win. Flip a coin 10 times, for example, and the number of heads you obtain could realistically be anywhere from one to 10. Flip it 100 times, though, and you are very unlikely to acquire more than 70% of either heads or tails. Flip it 1,000 times, and it is a virtual certainty that you will have flipped no more than 60% of heads or tails (and much more likely, less than 55%).
This predictability through which the universe manifests itself is not irrelevant to football. Two-point conversion rates and onside kick recovery percentages, for example, remain relatively stable from year to year. There may be blips in the data from time to time, but the overall statistics always (always!) regress back toward the mean.