By: Edward Egros

3rd

The Truth About 3rd Down

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Anyone paying attention to stats during an NFL broadcast has noticed 3rd down conversions being reported. It is an easy way for commentators to critique how clutch a team is and if an offense can maintain a drive when the pressure is at its peak. Obviously a team converting on 100% of its 3rd down attempts is probably winning the game, but otherwise it is not nearly as helpful a statistic as suggested.

For this exercise I took 10 seasons' worth of NFL data (2007-2016) and looked at conversion rates for 1st down, 2nd down, 3rd down and the number of regular season wins that team accumulated. Logically, it would make sense to have an increasing percentage with later downs because you often have fewer yards to go before moving the chains. The numbers reflect this trend: on 1st down, teams on average convert 20% of the time, on 2nd down it's 30.3% and on 3rd down it's 38.1%.

To make things simple, I then calculated a linear regression, treating wins as my dependent variable and keeping it continuous
so as not to lose information. Here are the results:

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As expected, every down is significant to wins at the 99% level, because the more you convert, the greater your chances of success. The degree to which each down matters does go up, as reflected by the coefficients increasing with each successive down. And, even though later downs should be easier to convert, the coefficient is still increasing, perhaps suggesting third down conversions do matter more than first and second.

However, the
R-squared and adjusted R-squared only hover around 28%. In other words, conversion rates only account for 28% of why a team wins or loses, so a 3rd down conversion percentage by itself is less that figure (22% if 3rd down rate is the only explanatory variable). While these rates are statistically significant (especially on 3rd down) they are also noisy.

In previous blog posts, I have outlined which factors best determine the outcome of football games (
and they are detailed in my Cowboys data visualizations). One reason why I never brought up 3rd down conversion rates is because of how noisy the variable is and how it takes away from 1st and 2nd down. Many others have their own ways of determining success based upon the down, but also the distance. I would suggest, for sake of ease, promoting the discussion of 1st and 2nd down success rates, both as a pair, but also as a bridge to what is a reasonable 3rd down to convert when those plays occur.