Friday, September 19, 2008

Two Point Conversions

Last Sunday in the 2nd week of NFL season the Denver Broncos defeated the San Diego Chargers in an exciting 39-38 game. The Broncos make a controversial decision to go for a 2 point conversion down 38-37 with 29 seconds left, declining to take the 1 pt extra point for the tie.

Disregarding the broncos successful attempt - the question is: what was the statistically correct move?

-Most NFL fans and analysts would say the extra point - based on the fact that the broncos would tie the game and have a 50% chance of winning in overtime. This is basically the conventional wisdom. “As a general rule, I feel like I have an obligation to my team to give them a chance to win the game in overtime by kicking an extra point,” Jeff Fisher, the Tennessee Titans’ coach, said, “not by winning or losing the game on one play.”

However, the alternative (the 2 point conversion) has had a varying success rate since it's existence. This New York Times article describes this rate, including this interesting fact: "Last season, N.F.L. teams converted just 30 of 61 attempts, a paltry .492 success rate". (This rate has varied since the 2-point it's existence, but has been more successful lately possibly because of decrease incidence and better play calling). The same article mentions the success rate of an extra point is slightly below 99 percent.

Another scenario to account for is the broncos missing the 2 point conversion, kicking an onside kick (10-15 percent success rate), then scoring in the final ~30 seconds (probably a 20-30 percent assuming successful onside kick). This would indicate an additional 1-3 percent increase in the win probability for the 2-point conversion.

It is hard to take every variable to account including each player on the field, but I estimate the 2-point conversion was the correct statistical call by a very small margin. This is sometimes difficult to comprehend, Since it's a dichotomous result. Close to 50 percent of the time choosing the 2-point conversion will be fail and be the "wrong" chose.

“Sometimes you have to go with your gut,” Shanahan said. “I just felt like it was a chance for us to put them away. I didn’t want to count on the coin flip. I wanted to do it then, and obviously it worked out.”

While, I am not a fan of the phrase "going with your gut" (did his lunch make the decision?), I must commend Shannhan for taking this risk. Most coaches (like Jeff Fisher) tend toward be more conservative, choosing the statistically incorrect decision when it has high risk that could be later be blamed on the coach.

I admire Mike Shanahan for his longevity, unique use of "skill players" like running backs, and his ability to lay himself on the line by taking risky decision that is statistically accurate. Just not his post-explanations. ;)


Bo said...

Shanahan should be rewarded for saving the nfl from having another overtime game. The sudden death format in the nfl has to the worse thing about the game of football. A high scoring game like the chargers Broncos game will be decided by a coin flip over half the time. If I were a head coach and had a decent offense I would go for two. It definitely depends on what type of team you have, but coaches will probably choose the extra point in almost all of those situations. For all of the praise Shanahan received, he would have received twice of the amount of criticism if the Broncos would have lost the game.

allovertheeowl said...

Mike Shananananananannanananahanahanhanananahanahnahanananahnan

Dan said...

agreed the playoff system in the nfl stinks. i'm not sure i like the college that much either.