From 2002 to 2021, NFL teams converted 38.9% of all third down attempts. Third down performance is really meaningful when it comes to winning games, but it can also be pretty random from sample to sample. So as a result, third down performance has an outsized performance on who wins and loses that game, but is probably not all that predictable as to who will win the next game.

I thought it would be interesting to look at this in the context of the pre-game point spread. Let’s start with a few basic numbers, looking at this 20-year period.

• Teams that were favored by 1 to 5.5 points won 58.7% of their games.
• Teams that were favored by 6 to 8 points won 73.6% of their games.
• Teams that were favored by more than 8 points won 83.4% of their games.

But let’s say you know that the favorite would lose the third down battle. How does that change things?

• Teams that were favored by 1 to 5.5 points but were worse on third downs won only 43.4% of their games.
• Teams that were favored by 6 to 8 points but were worse on third downs won only 54.2% of their games.
• Teams that were favored by more than 8 points but were worse on third downs won 68.2% of their games.

Now, saying an underdog just needs to win the third down battle is not very helpful, and only a little more precise (and about as useless) as saying they just need to score more points. But it does help to provide some guardrails about the magnitude of third down performance. It can flip a big favorite into a coin flip, and a huge favorite suddenly has a real chance of losing.

Can we quantify exactly how important third down success is? I’m glad you asked. As we know, each team has a 38.9% chance of converting an average third down. Suppose each team has 15 third down attempts in the game. Let’s say one team coverts 10 of 15, while the other only converts five opportunities. The expected number of third down conversions for both teams is 5.8 (0.389 multiplied by 15), so one team converted 4.2 more first downs than expected, while the other converted 0.8 fewer than expected. The net difference, of course, is five conversions — let’s call that the net third downs added. [continue reading…]