By: Edward Egros

conversion

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.

Yes! Go for Two!

unknownIt's an odd feeling for football fans. After scoring a touchdown, the exhilaration must be contained just as quickly as it erupted, as this same offense, grinding down the field and travailing through the defensive puzzles presented, decides to go for two. The decision is rare: during the 2015 NFL season, 1,217 extra points were attempted, but only 94 times did a team go for two (7%). In fact, five teams never attempted a two-point conversion.

Pittsburgh Steelers quarterback Ben Roethlisberger suggested this week his team should go for two, every time. Though his team attempted more two-point tries than anyone else, fewer than one-fourth of the time did the Steeler offense return to the field after a touchdown.

Traditionally, this idea is irreverent. But analytically, this idea carries merit. Because 94% of extra points were converted last year, if a team always goes for two, they only need to convert 47% of the time to push. It is worth noting, a defense can return the football the length of the field for two points no matter what is being attempted. Though this happened only once and during an extra point, it could fractionally affect this expected value even if it statistically insignificant. Lifetime, teams convert their two-point attempts roughly 50% of the time, almost exactly what they need for it to be a push.

So why always go for two if it is a push and risk injury to more valuable players? And, perhaps more importantly, would this 50% success rate hold if teams went for two more frequently? Aside from the fact there is an obvious trend NFL offenses are improving and kickers are worsening (mainly because the distance of an extra point was moved back 15 yards), the following chart illustrates two-point tries:

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As expected, the 50% success rate remains relatively consistent regardless of how many times teams go for two. However, as stated before, this is a small sample size compared with the number of times a team could have gone for two, but elected for the extra point. Usually teams go for two when almost absolutely necessary. When it is not absolutely necessary, will the success rate be the same?

It's worth finding out.