Ewing: Kickers

Last Updated: 1/18/2015 8:58 PM ET
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How confident can we be in a kicker's field goal percentage?

Kickers are drafted each year. Since 2000, NFL teams have spent 31 draft picks on place kickers. The odds are a team will draft a kicker this year. We could argue the merits of drafting a kicker but we will leave that for another day. Instead, let's focus on how difficult it is to evaluate a kicker and determine which, if any, of this year's crop are worth drafting.

How confident can we be in a kicker's field goal percentage? The answer, not very. The problem is sample size. Last season Arizona State's Zane Gonzalez attempted 30 field goals, the most in Division I. There were 165 kickers that attempted at least one field goal last season; the average kicker attempted 14 field goals and the average field goal percentage was 68%.

Even as a four-year starter, a potential draftee is likely to have less than 100 attempts in his college career. In comparison, two-year starter and potential No. 1 draft pick, Johnny Manziel, has attempted 863 passes. None of the kickers that are likely to be drafted this year have 100 career attempts. With a small sample size it becomes difficult to ascertain a kicker's true field goal percentage.

For example, Chris Boswell (Rice), the No. 1 rated kicker by both ESPN and CBS, has attempted 88 field goals in his four-year career. Boswell has made 65 of those attempts (73.9%). Anthony Fera (Texas), another top rated kicker in this year's draft has connected on nearly 84% of his field goals but has attempted less than half (43 career attempts) the number as Boswell. Which kicker is better?

That is a difficult question to answer for many reasons but the most important would be the difference in number of attempts. To account for the difference in sample size and compare both kickers based on field goal percentage we can construct confidence intervals.

To do this analysis I used R: Test for Equal or Given Proportions, similar to the analysis done by Kevin Rudy in his sample size analysis for kickers. The drawback of this analysis is that it assumes the probability of each field goal is the same, which is incorrect. Distance, weather, and other factors impact the probability that a kicker will make a field goal. The purpose of this analysis is to show how much variance exists in small samples, not be an accurate predictor of a kicker's field goal percentage.

Based on this analysis, a 95% confidence interval for Boswell's field goal percentage would be (63.2, 82.4). This means that Boswell's true field goal percentage, with 95% confidence, is between 63% and 82%. The same analysis for Fera produces a 95% confidence interval of (68.7, 92.7). On the high-end Fera's field goal percentage could be greater than Boswell's, but there is a wider range of outcomes due to the smaller sample size. Thus, while Fera currently has a higher field goal percentage, his true field goal percentage could be significantly lower.

Below are the 95% confidence intervals for five of the top kickers in this year's draft.

Kicker Attempts Makes FG% 95% Lower 95% Upper
Chris Boswell 88 65 73.9 63.2 82.4
Anthony Fera 43 36 83.7 68.7 92.7
Cairo Santos 78 61 78.2 67.1 86.4
Zach Hocker 79 61 77.2 66.1 85.6
Jeff Budzien 55 48 87.3 74.9 94.3

It is not just accuracy but also leg strength that is important to NFL teams. When you start to evaluate accuracy on long field goals, 40+ yards, the sample sizes become even smaller and the variance in possible outcomes for the kicker's true field goal percentage increases.

Below are the 95% confidence intervals for field goals over 40 yards.

Kicker Attempts Makes FG% 95% Lower 95% Upper
Chris Boswell 52 33 63.5 48.9 76.0
Anthony Fera 16 10 62.5 35.9 83.7
Cairo Santos 38 24 63.2 46.0 77.7
Zach Hocker 30 19 63.3 43.9 79.5
Jeff Budzien 17 11 64.7 38.6 84.7

The wide confidence intervals for each kicker illustrates how difficult it can be to evaluate kickers for the NFL Draft. The sample sizes are small; therefore, possible outcomes for the true field goal percentage are large.

When evaluating this year's group of kickers, a team would be wise to select Chris Boswell. Not because he is definitively the best kicker available, but rather, he is the kicker whose future performance is most likely to reflect his collegiate career. The variance in his field goal percentage on all attempts as well as those over 40 yards is the smallest among the group.