Monday, September 22, 2014

No league for old men


Back in December of last year I went through some calculations with the data set I stitched together including both performance and salary data. You can check the post out here for a series of simple graphs that go a long way toward explaining team behaviors and why certain things are the way they are in the NFL, particularly as they relate to player tenure and performance over time.

As I was reading through that post again recently, however, I was struck that it makes the implicit assumption (explicit above) that things are the way they are. What I mean is that this data is the average over a period of time, while there may in fact be some trends to observe by looking at the period year-by-year.

Charts



The average age of all NFL players[1] fluctuated between 26.5 and 27 for most of the post-1994 period (the salary cap era) before dropping from 2008 through 2013 almost without interruption. Applying a weight based on games played or games started, the other two lines on the chart, indicate that the pattern is consistent for the starters as well as the backups. NFL rosters are roughly half a year younger than they were from 94-08.


Even the composition of the most elite players has trended younger through the period, and while the smaller sample size manifests itself in larger year-to-year changes the overall effect seems stronger than the ~0.5 year change for all players.



Ok, why?



With the case for players getting younger fairly well established, it seems appropriate to move into speculation as to why this is happening. There are two theories that come to mind.



First, players are entering the league younger and so driving down the age of each cohort. Since football tends to be hard on the body tenure ends up mattering more than age and younger entry pulls the average age down as long as the incoming players keep coming in earlier.


As the chart indicates, the data shows first-year players have been indeed getting younger (please check the footnote for me to walk this back)[2]. The trend is clearly downward but the overall change is not huge even when you add a trendline that smooths out the variations. The fact that it is similar to the 0.5 years that the average of all players changes is a coincidence – for the 0.5 year change in the average of all players to be explained by this it would have to be either much larger or much earlier, because the change in incoming age takes years to filter through with older cohorts still included in the total.

The other theory I have ties to the salary cap. NFL teams sign players to contracts that specify a certain number of dollars[3] over a certain period of time. They also have to stay under the salary cap that the league sets on an annual basis depending on revenues and a rather complicated formula negotiated as part of the NFL’s collective bargaining agreement with the NFLPA. In the event that the cap rises slowly or shrinks teams may be put to a decision on whether their most expensive players are a good value while a rising cap allows problems to be pushed out into the future.


From this chart, with the average ages supplemented by the % change in the salary cap from year to year, it appears that the changing cap has a significant effect.


Looking at just the relative changes and singling out the average by games started – the metric most likely to be affected by cap-influenced decisions because younger players are much less likely to be starters and much more likely to be low-paid – the relationship is striking. The two spikes in average age correspond closely to the spikes in the salary cap. Teams who might otherwise have been forced to rebuild were able to hold on to marginal veterans. There are limits to the correlation, however, as big spikes in average age appear to lead to future decreases even when the cap rises at a decent rate (1999, 2007-8), suggesting that cap flexibility can only go so far before teams make the same decisions for other reasons such as performance.

There could of course be other explanations such as increased injuries (or awareness thereof), but the precipitous drop of the past few years seems to me to have been strongly influenced by the flat salary cap.




[1] All data sourced from pro-football-reference.com, pulled April 2013 for 1994-2012 and June 2014 for 2013. Salary cap for 2010 projected same as 2009 even though 2010 had no cap (try telling that to Washington or Dallas).

[2] I am not 100% comfortable with the data here. I have a feeling that PFR is more complete the more recent we get, in that it includes more roster players who never actually see the field. Weighting the data instead by games played would skew the data based on skill of the incoming class and end up giving us wide variations year to year. Anyway, this seems like the best available cut at it, but I wouldn’t recommend using this as the single source for any big claims.


[3] Just stay with me, this is going somewhere.

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