Judging Kickers

_The NFL playoffs are starting, and kickers often end up playing the role of hero or goat by making or missing a clutch kick. I thought it’d be worth it to re-post something I wrote back when the [Saints lost to Arizona][1] earlier this year. Saints kicker Garrett Hartley was excellent in the playoffs last year, but got off to a rocky start this year. After the jump, I argue that it’s virtually impossible to judge a field goal kicker’s accuracy with any certainty. Gives you a lot of confidence, doesn’t it?

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The Saints’ special teams struggles are well-documented. However, it’s really difficult to tell whether the missed field goals are the result of poor kicking or poor luck. Judging kickers is really difficult because of sample size issues. In most years, a kicker will attempt between 20-40 field goals. This sample is too small to tell whether a kicker’s success (or lack thereof) is due to skill or luck. Let’s look at a quick example to show what I mean.

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Let’s say that we have a robot kicker that is 80% accurate on field goal attempts (we’ll ignore the effects of field goal distance for this example). That’s a very good success rate. If that kicker attempts 30 field goals in a year, then the kicker has about a 51% chance of making 23, 24, or 25 of those field goals and a about a 76% chance of making between 23 and 30 of the field goals. However, the kicker also has about a 24% chance of making 22 or fewer field goals, and about a 6% chance of making 20 or fewer field goals.*

*Statistical note for nerds: I’m considering kicks to be Bernoulli trials here with n= 30 and p=0.8. Disagree? Tell me what I’m doing wrong in the comment thread.

In other words, in any one year, there’s about a 1 out of 4 chance that even a good field goal kicker will miss a whole bunch of field goals. And, even if a kicker is having a good year, he may miss several field goals in a row. That’s just the way that probability goes. That’s why judging a kicker is very tough, and I’m a bit indifferent on the Hartley-Carney debate. I do wonder about kickers’ confidence levels and whether they influence field goal percentage, but, given the small samples, there’s no good way to study it.

As one of my dissertation committee members phrased it: Probability’s a bitch. It’s really hard to tell if a kicker’s good or bad. The real secret is to score touchdowns and let the other team worry about their kicker.

[1]: http://www.whodatreport.com/2010/10/cardinals-30-saints-20/