Shrinkage estimation of NFL field goal success probabilities
Abstract
National Football League (NFL) kickers have displayed improvement in both range and accuracy in recent years. NFL management in turn has displayed a rather low tolerance for missed field goals, particularly in gamedeciding situations. However, these actions may be a consequence of a perceived appreciable variability in NFL kicker ability. In this paper, we consider shrinkage estimation of NFL kicker field goal success probabilities. The idea derives from the literature on estimating batting averages in baseball, though the classic JamesStein shrinkage approaches there do not apply to independent binary field goal attempt trials. We study a variety of weighting schemes for shrinking modelbased kickerspecific field goal success probability estimates to a leaguewide estimate, as a function of distance. As part of the development, we briefly detail collecting NFL playbyplay data with the R statistical software package, identify the complementary loglog link function as preferable to the more commonly applied logit link function in a generalized linear model for field goal success, and demonstrate the desired variancereduction, both in and out of sample, enjoyed by our proposed shrinkage estimators. We illustrate our methods by ranking NFL kickers from 1998 to 2014, analyzing individual kicker success at midrange and longrange field goal attempts, and studying kicker ability over the last decade. Stadium effects are added to the model and found to be highly significant and to have an impact on the kicker rankings.
1Introduction
Given the high frequency of personnel transactions involving field goal kickers in the National Football League (NFL), there appears to be a perception of considerable variability among their abilities to carry out their duties, at least among general managers. In particular, when kickers miss field goals or extra points that are perceived to cost their team a game, they are sometimes terminated shortly thereafter, presumably reasoning that a replacement would have a better chance of success from the same distance. Consider the 2010 New Orleans Saints. In Week 3, Garrett Hartley missed a 29 yard field goal attempt in an overtime loss. He was replaced by veteran John Carney, who in turn missed a 29 yard field goal attempt in another loss two weeks later. Hartley then regained his job the following week. The Saints are not the only team with a quick trigger on inseason kicker personnel decisions. Table 1 summarizes the number of kickers who have been released, for reasons not listed as injury, over the 20002014 timeframe. On average, there are more than 3 such kickers released per season. These counts were assembled by inspection of season summaries of kickers found on and subsequent posthoc inspection of various websites that track transactions.
Table 1
Year  20012003  20042006  20072009  20102012  20132015 
Number players released  12  10  11  6  10 
Quantitative analyses (Morrison and Kalwani, 1993) have found surprisingly limited evidence of differences among kickers. In an investigation of weather and distance effects on kicking, Bilder and Loughin (1998) chooses not to model individual kicker effects, describing kickers as “interchangeable parts.” In a study of whether “icing the kicker” affects kickers (it does!), Berry and Wood (2004) also assumes that dependence of logodds of success on distance to be the same for all kickers. If there are differences among kickers in the way they are affected by field goal attempts of increasing distance, these effects may be subtle enough to go undetected without large samples. Due to these subtleties, estimation of success probabilities for particular kickers can be a difficult task.
We consider the problem of estimating probabilities of field goal success using playbyplay data available on the web. As with analogous investigations in the literature, we develop generalized linear models which assume kicks to be independent Bernoulli trials. However, we show that the complementary loglog (CLL) link provides a better fit than the logistic regression models typically applied in prior work. In models that include kicker effects, estimates of field goal success for inexperienced kickers may have high sampling variance due to small samples. We investigate a number of shrinkage techniques that borrow information from field goal attempts by other kickers in an effort to reduce the sampling variance in these kickerspecific estimates.
The textbook by Morris and Rolph (1981) uses the problem of estimating field goal success probabilities after regression on distance (grouped into intervals with 10 yard widths) as an introduction to logistic regression. Berry and Berry (1985) develops a generalized linear model for field goal and extra point success probabilities using a customized link function that is derived from consideration of three factors: distance, leftright error, and the chance of a nongoalpost related error (a fumble or blocked kick). Other work investigating additional factors beyond distance that may be associated with field goal success probabilities, such as weather and psychological pressure, includes Bilder and Loughin (1998); Pasteur and CunninghamRhoads (2014); Berry and Wood (2004); Clark et al. (2013). All of these contributions are in agreement that the single most important factor in predicting field goal success is the distance of the attempt. Morrison and Kalwani (1993) fails to reject the hypothesis of equal success probability among all kickers. Pasteur and CunninghamRhoads (2014) uses crossvalidation to select variables and finds that adding distance to an interceptonly model decreases a predictive mean squared error criterion by 14%, but that adding an additional 7 variables (including temperature, wind, kicker fatigue, defense quality, and whether the kick was in Denver) brings about a further reduction that is less than 2%. Since many failed extra point attempts are a consequence of poor snaps or holds, we restrict our attention to field goal attempts and do not consider extra point success probabilities or data in our analyses.
Our aim in this paper is not to select variables for the best model, nor to account for the various subtle (in comparison to distance) effects of weather or psychological pressure, but rather to demonstrate the potential advantages of shrinking regressionbased estimators towards a central value with the goal of variancereduction. Indeed, this work is motivated by the success of JamesStein shrinkage techniques (Efron and Morris, 1975; Brown, 2008) for the problem of predicting batting averages in baseball.
In Section 2, we describe our data collection procedure and consider datadriven selection of the link function in a generalized linear model for field goal success. In Section 3, we consider a variety of shrinkage estimators for the kicker and distancespecific success probabilities as functions of model parameters. In Section 4, we illustrate several applications of our shrinkage estimators, including a ranking of kickers and a discussion of long field goal attempts and stadium effects. We present a summary and conclude in Section 5.
2Data and statistical models
2.1Data collection: web scraping with R
Like most other work in the area, we assume kicks to be independent and model the dependence of success probability on explanatory variables using generalized linear models. The explanatory information we consider includes only the distance of the attempt and who the kicker is. Using the readLines() command and regular expressions functionality in the R statistical software package (R Core Team, 2014),, the distance of a field goal attempt is gleaned from the website with url. This website has complete playbyplay information going back as far as 1998. More specifically, the string “yard field goal" is matched and the kicker and distance occurring in the string before the match are recorded along with the outcome that follows the match. Every field goal attempted, even those on which a penalty occurred, but for which a result was given in the playbyplay transcript, are included in the dataset. The task loops over seasons 19982014, weeks within season (including the NFL playoffs), games within week, and finally plays within game.
The dataset contains 17, 104 attempts from more than K = 111 kickers. Kickers who made or missed all of their career attempts are excluded from consideration to avoid separability issues with maximum likelihood estimation. (Jaret Holmes played for three teams from 19992001, making all five of his field goal attempts and all four of his extra point attempts successfully. Danny Boyd was even better, making 5/5 field goals and 7/7 extra points for Jacksonville in 2002.)
Outcomes were dichotomized as “good” or “not good”, the latter event includes blocks and fumbled holds. Our reasoning is that the blame for failed attempts usually rightly falls on the kicker, but sometimes the snap, the hold or the blocking are at fault. The records online do not easily allow for determination of blame. An example of an entry that was scraped from the url (http://www.profootballreference.com/boxscores/199809060cin.htm). (a game from week 1 of the 19981999 season) appears in Fig. 1.
Fig.1
Occasionally, comparisons between total kick frequencies computed from playbyplay data with kicker career totals will reveal slight discrepancies, where certain plays are absent from the profootballreference playbyplay information. For example, in a 1998 Week 1 game of the New England Patriots at the Denver Broncos, Adam Vinatieri of the New England Patriots had a 37 yard field goal attempt during the second quarter. The profootballreference playbyplay account of the event is blank (see Fig. 2). We resorted to the site to attempt to rectify this particular record. Though we could not find links on the site to playbyplay information for games before 2001 (a situation in which automated screenscraping is then difficult), we were able to find some urls, with links provided by a search engine.
Fig.2
2.2Choice of a link function
For notation, we let k = 1, …, K index the kickers (K = 111 in our application) and let j = 1, …, N (N = 17104) index all of the field goal attempts. For each attempt, we let binary outcome O_{j} = 1 if the kick is good, and O_{j} = 0 otherwise. The probability of success of attempt j is denoted π_{j}. The general form for the loglikelihood function of all of the probability parameters {π_{j}} for independent, dichotomous data with outcomes o_{j} is given by
Generalized linear models adopt a link function g, through which the probabilities π_{j} are transformed to an expression that is linear in regression coefficients introduced to model the effects of available explanatory variables (and reduce the dimension of the parameter space). We define d_{j} as the distance of the jth attempt and define indicator variables for kicker k as X_{jk} = I (jthattemptisbykicker k), for k = 1, …, K, with regression coefficients β_{1}, …, β_{K}. The probability parameters are now, after transformation by the link function g, linear in the regression coefficients,
We refer to β_{k} as a kickerspecific slope and can test the significance of estimated regression coefficients by comparison with a generalized linear model with only one slope, β, to accommodate all kickers,
In applications in which statistical regression models are developed for binary response variables, logistic regression seems to be the most popular choice (Pasteur and CunninghamRhoads, 2014; Morris and Rolph, 1981; Bilder and Loughin, 1998; Berry and Wood, 2004), perhaps because of its interpretability, particularly through odds ratios. However, several other link functions are easily explored using statistical software for generalized linear models. We introduce a superscript notation for probability functions associated with kickers that will simplify the shrinkage estimation exposition in Section 3. Let π^{k} (d) denote the success probability function for kicker k at any distance d. We consider three choices for the link function: logistic, probit, and CLL. Let Φ and Φ^{1} denote the cumulative distribution function and quantile function, respectively, of a random variable with standard normal distribution. Then the links and inverse links (probability functions) for these three candidates are given below:
The loglikelihood function of a specified link function uses the reparameterizations above. As an example the loglikelihood for the logistic link is given by
Table 2 summarizes the maximized loglikelihood function when using the logit, probit, and CLL link functions for estimating field goal success probability in our data set. Tests comparing the nested models within a column of the table are all highly significant on 110 degrees of freedom (comparing models with K = 111 different slopes to a model with a single slope), indicating significant differences among the estimated slopes. With such a large sample, there does appear to be enough data to detect potentially subtle differences among kickers.
Table 2
Link function  
Comp.  
Data  Model  Logit  Probit  loglog 
All  Common slope  14382  14365  14367 
Kickerspecific slopes  14081  14062  14060  
χ^{2} (df = 110) statistic  301  303  307  
Restricted to  Common slope  11827  11820  11815 
25 ≤ d ≤ 50  Kickerspecific slopes  11547  11535  11524 
χ^{2} (df = 110) statistic  302  307  312 
Figure 3 presents the inverse link functions applied to the empirical success frequencies against distance. The lines overlaid on the plots are from maximum likelihood estimation of the singleslope model. Residuals computed as standardized differences between transformed success frequencies and the estimated link functions are plotted against distances in Fig. 4, providing an enhanced visualization of the fits. Linkspecific lowess smoothers overlaying these plots suggest that the residuals from the CLL fit exhibit the least dependence on distance.
Fig.3
Fig.4
In an analytical comparison of link functions, McCullagh et al. (1989) observes that for trials with intermediate success probability, 0.1 ≤ π ≤ .9 (the vast majority of field goal attempts), probit and logistic functions are nearly linearly related, so that discrimination between the fits of models using these two links is difficult. Additionally, for trials with success probability approaching one (which occur as field goal attempt distances decrease towards 17 yards), the CLL approaches one more slowly than either of the other two link functions. The CLL link can also differ (Dobson and Barnett, 2011) from logit and probit links for small values of π, which are often of greatest interest when assessing kickers. Close inspection of Fig. 3 and Fig. 4 seem to reflect these observations, with better agreement with empirical frequencies on short kicks than that provided by either logit or probit. Fitting these models using kicks at the extremes may be problematic in that misses at short distances are likely due to mishandled snaps and attempts at long distances may lead to selection bias since these attempts are probably only afforded to very good kickers. In light of these issues, loglikelihood ratios are also reported after restricting attention to kicks with distances between 25 and 50 yards. With this restriction, the CLL link achieves the maximum likelihood among the link functions for models with either a single slope or with kickerspecific slopes.
The statistic proposed by Hosmer and Lemeshow (2004) (HL) offers another way to assess goodnessoffit in regression models for binary data with continuous explanatory variables. The HL statistic is a Pearsonstyle measure of goodnessoffit whose sampling distribution under a correctly specified model can be approximated by a chisquare distribution. The HL procedure partitions all kicks into 10 groups of roughly equal size. (We accomplished this using the algorithm laid out in the documentation for PROC LOGISTIC within the SAS statistical software package (SAS Institute, 2011). The procedure then computes, for each group, observed (O_{i}) and expected (E_{i}) counts of successful kicks by averaging estimated probabilities within group, in order to compute the χ^{2} statistic, X:
The summary statistics necessary for computation of the HL statistic are given in Table 3 for the logit and CLL links. The logistic regression model does not provide a good fit for the 10th group, where the estimated success probability is highest. The average estimated probability among these 1708 kicks is
Table 3
Complementary loglog link  Logit link  
FG Made  FG Made  
Group i  kicks, n_{i}  o_{i}  e_{i}  χ^{2}  kicks, n_{i}  o_{i}  e_{i}  χ^{2} 
1  1710  883  898.02  0.53  1711  871  842.32  1.92 
2  1699  1112  1097.46  0.54  1712  1132  1104.61  1.91 
3  1709  1217  1213.65  0.03  1715  1218  1241.27  1.58 
4  1712  1315  1310.01  0.08  1705  1334  1336.55  0.02 
5  1706  1387  1392.39  0.11  1710  1385  1425.98  7.09 
6  1711  1473  1478.97  0.18  1712  1470  1500.63  5.06 
7  1720  1565  1558.60  0.28  1709  1555  1555.20  0.00 
8  1709  1604  1606.43  0.06  1709  1600  1598.85  0.01 
9  1713  1655  1653.44  0.04  1713  1655  1635.25  5.26 
10  1715  1685  1685.86  0.06  1708  1676  1655.34  8.37 
sum  17104  13896  13894.83  1.89  17104  13896  13896.00  31.23 
Table 4 gives HL chisquare statistics andpvalues on 8 degrees of freedom for each of the three link functions with either a single slope, or with kickerspecific slopes. The models with the CLL link function are the only ones that do not exhibit significant lackoffit. With such a large sample size (N = 17, 104 attempts), even subtle lackoffit is likely to be detected, as in the logistic (p = .0291,. 0657) and probit (p < .0001, p = .0001) models. Despite this significance, neither of the two provides a bad fit.
Table 4
Link function  
Data  Model  Logit  Probit  Comp loglog 
All  Common slope  34.1(<0.0001)  17.1 (0.0291)  8.9 (0.3489) 
Kickerspecific slopes  31.2 (0.0001)  14.7 (0.0657)  1.9 (0.9843)  
Restricted to  Common slope  16.8(0.0329)  11.6(0.1720)  7.9(0.4463) 
25 ≤ d ≤ 50  Kickerspecific slopes  32.3(0.0001)  24.1(0.0022)  14.8(0.0630) 
Restricted to  Common slope  19.4(0.0130)  12.4(0.1326)  7.8(0.4508) 
25 ≤ d ≤ 55  Kickerspecific slopes  24.2(0.0021)  16.3(0.0380)  10.4(0.2383) 
Lastly, a link function could be chosen on the basis of how well probabilities estimated using the link can predict successful and unsuccessful field goal attempts. A decision rule which predicts a successful field goal when the estimated success probability exceeds some threshold (
Table 5
Link function  
Model  Logit  Probit  Comp loglog 
Common slope  0.7490  –  – 
Kickerspecific slopes  0.7640  0.7641  .7646 
Since the CLL link enjoys the smallest HL and largest AUC under crossvalidation (as described in Section 3.1), computations under the other two link functions are excluded from presentation in the remainder of the paper.
3Shrinkage estimation
Modelbased estimates of kickerspecific success probabilities have large variance except in cases where the number of observed attempts for a given kicker is very large. As an example, consider interval estimation of the probability of success when kicking a 45 yard field goal using the CLL link function. For Adam Vinatieri, who has 572 career attempts, the 95% interval estimate is (0.69, 0.78). However, the 95% interval estimate of 45 yard field goal success probability for Garrett Hartley, who has 115 career attempts, is (0.59, 0.80), more than twice as wide as the interval estimate for Vinatieri. Estimators with reduced variance may be constructed by taking a weighted average of the kickerspecific maximumlikelihood estimator (MLE) and one developed for the average of all kickers. That is, the variance of an estimator may be reduced by shrinking it towards another estimator which has much lower variance.
Efron and Morris (1975) and Brown (2008) employed this method, viewed from an empirical Bayes perspective, to predict endofseason batting averages in Major League Baseball. In particular, the papers propose shrinking hitterspecific estimates based on a small number of atbats towards the early season batting average among all hitters. The methods are based on the theory James and Stein (1961) develops for estimation of the mean of a multivariate normal distribution. That theory does not apply directly to estimation of success probabilities for independent binary trials, as in field goal attempts, but the idea of variancereduction is worth exploring.
Without any shrinkage, the kickerspecific MLE of the success probability at a given distance d is given by
The estimator with complete shrinkage is simply the empirical frequency of all kicks taken by all kickers at that distance,
These empirical frequencies are plotted in the top left graphic in Fig. 3.
3.1Proposed shrinkage estimators
For shrinkage estimators of the success probability at a given distance, π (d), we consider estimators that are weighted averages of the MLE and empirical frequency estimators defined above,
The first shrinkage estimator we propose is simply the midpoint between the two components. We refer to this shrinkage estimate, obtained by setting ω = 1/2, as the midpoint estimator (denoted by superscript M):
One approach towards constructing a shrinkage estimator that weights modelbased kickerspecific estimates more heavily is simply to weight components in inverse proportion to their standard errors. This will upweight the MLE,
Note that because sample sizes differ, this weighting scheme can lead to different weights at different distances.
Another approach is to choose a weighting function for the MLE that is increasing in n_{k} towards a maximum weight of one. Any cumulative distribution function (CDF) has this property. We choose the exponential distribution function, which for a suitably chosen rate parameter a, increases rapidly for small n with diminishing returns on large n_{k}:
This choice leads to the exponentially weighted shrinkage estimator,
The quantity a in the exponential weights estimator can be viewed as a tuning parameter, whose value is “fit” using the entire dataset and whose performance is evaluated by crossvalidation (see Section 3.2). For the a parameter, mean values 100 < a < 700 were considered, reasoning that the kick counts fell in the interval 4 < n_{k} < 572. A search of values of a, to the nearest multiple of 10, achieves a HosmerLemeshow (HL) statistic of HL = 9.32 at an optimized value of a = 670. Using the exponential CDF with mean a = 670 for a weight function, the corresponding weights on the modelbased estimates for kickers with sample sizes of n_{k} = 10, 50, 100, 200, 400 would be 0.02, 0.07, 0.14, 0.26, 0.45, respectively.
Figure 5 presents a plot of the weight function, ω for these different shrinkage estimators. The two separate curves provided for the IV weighted estimator corresponds to kicks at d = 45 and d = 55 yards. The empirical frequencies at these two distances were
Fig.5
The estimators of success probability can be naturally shrunk towards central values by means of a Bayesian generalized linear model with CLL link utilizing empirical Bayes estimation (see e.g.,Carlin and Louis (1997)). In such a model, as fit using the bayesglm function of the arm package in R (Gelman et al., 2014), the regression coefficients are assumed to be independent, normally distributed random variables with mean and variance characterized by hyperparameters that are specified from data, in our case:
The hyperparameters for the intercept are the default choices for the bayesglm function and those for the kickerspecific slope were chosen from the sample mean and variance of maximum likelihood estimates from the frequentist generalized linear model fit.
Fig. 7 illustrates the reduction in variance due to shrinkage. For attempt distances, d = 30, 40, 50, 60 yards, random samples of 10 kicks were selected. The estimated success probability estimates are plotted using different colors, with estimation technique on the horizontal axis. Not only is the reduction in variation apparent, but the modelbased estimates at 60 yards are shifted downward. The explanation for this shift is lackoffit of the model for very long attempts. Shrinkage towards the empirical frequency tends to alleviate bias caused by this poor fit.
Fig.7
3.2Assessment using crossvalidation
While HL is useful for quantifying how well several fitted models align with observed data, it may not tell the entire story as to how well the fitted model would generalize to observations made in the future, or outofsample data. Crossvalidation (CV) can be used to quantify the degree to which a model fitted to observed data generalizes to data not used to fit the model or to compare the capacity of several models to generalize. Pasteur and CunninghamRhoads (2014) uses fivefold CV to select weather, pressure, and fatigue variables for a multiple logistic regression model. In fivefold CV, the dataset is partitioned into five subsets of equal size. The first subset is held out of the process where the model is fit, and then the fitted model is used to estimate the probabilities of the events that were held out. HL is then computed only for the heldout subset. This step is repeated four more times, holding out each subset exactly once. As can be seen from inspection of the first two numerical columns of Table 6, neither the modelbased estimate (CLL) nor the empirical frequency (EF) generalize well under 5fold CV, in the sense that there is disagreement between the number of observed and expected successful kicks when attempts are put into the 10 groups for the HL statistic. Recall that the midpoint estimated probability for any kick is simply the midpoint between CLL and EF. While this shrinkage estimate has a greater HL than its CLL and EF components when the entire dataset is used both to fit the model and to compute the goodnessoffit statistic, it shows better agreement between observed and expected successful kicks under CV. The CV procedure was repeated four times, using four random partitions of the dataset into five subsets. Each time, the midpoint estimate enjoys a better fit for the heldout data than either of its two components, as quantified by the HL statistic. The shrinkage estimators based on the exponential weights, with a = 660, 670 or 680 also performed better than either component by itself. In computation of these estimates, the step of tuning the a “parameter” was not repeated during crossvalidation. Rather, estimates based on values of a = 660, 670, 680 (which did the best on the entire dataset) were computed for every observation, and only those results are reported here. If this weighting scheme is tolerable, then it appears to perform comparably to the simple midpoint shrinkage estimator. Among the six estimators reported in Table 6, there is no dominant choice when considering performance under crossvalidation. One clear observation, however, is that the midpoint and exponential shrinkage estimators are generalizing to outofsample data better than either the CLL or EF components by themselves.
Table 6
Shrinkage  5fold Cross Validation  
Estimator  All Data  1x  2x  3x  4x  CV avg 
CLL  1.89  15.60  20.13  22.59  22.13  20.12 
EF  0.00  8.96  25.25  19.45  13.14  16.70 
midpoint  11.87  8.70  9.57  5.29  4.68  7.06 
exp(a=660)  9.83  6.97  10.03  4.19  9.07  7.56 
exp(a=670)  9.32  7.05  10.52  4.23  9.21  7.75 
exp(a=680)  9.93  7.16  10.20  4.81  7.73  7.47 
IV  12.69  16.30  33.70  19.12  18.50  21.90 
Bayesian CLL  19.08  29.03  10.72  11.94  17.72  17.35 
4Applications
4.1Ranking kickers
In addition to estimating the success probability for a given kicker from a given distance, the estimation methodology can be used to rank NFL kickers. Historically, comparisons among kickers in sports media have been made using the overall number (as on espn.go.com) or proportion of successful attempts (as on sports.yahoo.com or si.com) without regard to distance, as it not obvious how to incorporate distance and degree of difficulty based on summary statistics. Alternatively, Berry and Berry (1985) suggests averaging modelbased kickerspecific estimates over the leaguewide distribution of attempt distances. A histogram for these attempts is given in Fig. 8. The average distance over all 17, 104 attempts was 36.6 yards. Tables 7, 8, and 9 present a ranking of kickers under this summary statistic, using several different methods of estimation, along with the standard “overall” metric.
Fig.8
Table 7
Kicker  avg attempt  <40  40  49  ≥50  pct  rank 


Justin Tucker  38.11  64/65  26/30  15/21  0.91  1  0.92  0.87 
Dan Bailey  38.71  63/66  35/39  17/25  0.88  3  0.90  0.86 
Chandler Catanzaro  38.21  17/19  10/11  2/3  0.88  5  0.89  0.85 
Kai Forbath  36.90  35/38  22/25  2/4  0.88  4  0.88  0.85 
Blair Walsh  39.01  53/57  19/24  17/24  0.85  17  0.88  0.84 
Connor Barth  38.52  65/69  42/51  13/20  0.86  11  0.87  0.84 
Cody Parkey  35.42  24/26  4/6  4/4  0.89  2  0.87  0.84 
Patrick Murray  41.08  9/11  6/7  5/6  0.83  27.5  0.87  0.84 
Dan Carpenter  38.03  111/115  65/82  20/34  0.85  16  0.87  0.84 
Steven Hauschka  36.45  95/100  40/49  9/17  0.87  8  0.86  0.84 
Greg Zuerlein  39.96  43/48  17/19  13/22  0.82  39  0.86  0.84 
Robbie Gould  36.55  164/173  74/100  17/23  0.86  9  0.86  0.84 
Rob Bironas  37.62  147/158  71/92  24/35  0.85  15  0.86  0.84 
Jason Hanson  38.31  210/223  108/141  43/68  0.84  23  0.86  0.83 
John Kasay  37.32  198/203  88/114  32/63  0.84  22  0.85  0.83 
Stephen Gostkowski  35.08  185/201  65/85  14/18  0.87  7  0.85  0.83 
Josh Brown  37.72  185/196  81/111  35/54  0.83  25  0.85  0.83 
Matt Bryant  35.91  200/214  73/95  20/34  0.85  12  0.85  0.83 
Mike Vanderjagt  35.85  157/167  71/95  15/23  0.85  13  0.84  0.83 
Matt Stover  34.96  236/247  97/131  9/20  0.86  10  0.84  0.83 
Jeff Hall  39.60  2/2  1/2  1/1  0.80  54.5  0.84  0.82 
Shayne Graham  35.29  197/211  69/89  15/30  0.85  14  0.83  0.82 
Sebastian Janikowski  38.99  222/239  111/148  48/88  0.80  53  0.83  0.82 
Phil Dawson  35.65  256/280  85/115  35/51  0.84  19  0.83  0.82 
Adam Vinatieri  35.80  322/354  135/173  26/45  0.84  18  0.83  0.82 
Jeff Wilkins  37.35  166/177  67/100  28/41  0.82  38  0.83  0.82 
Joe Nedney  36.77  154/163  62/88  17/31  0.83  35  0.83  0.82 
Ryan Longwell  36.68  228/248  97/133  24/40  0.83  32  0.83  0.82 
Josh Scobee  38.15  142/153  74/106  26/42  0.80  50  0.83  0.82 
Gary Anderson  35.70  105/114  56/74  3/7  0.84  20  0.82  0.82 
Ryan Succop  37.03  88/96  43/58  12/20  0.82  36  0.82  0.82 
Shaun Suisham  35.98  142/157  76/94  6/18  0.83  30  0.82  0.82 
Randy Bullock  38.66  34/37  19/25  5/11  0.79  57  0.82  0.82 
Nate Kaeding  35.46  129/135  51/73  10/19  0.84  21  0.82  0.82 
Garrett Hartley  35.12  62/73  28/34  6/8  0.83  24  0.81  0.81 
Jay Feely  36.40  232/259  95/130  19/34  0.82  41  0.81  0.81 
Nick Folk  37.33  128/142  57/78  19/34  0.80  52  0.81  0.81 
Table 8
Kicker  avg attempt  <40  40  49  ≥50  pct  rank 


Mason Crosby  37.47  155/168  56/76  23/50  0.80  56  0.81  0.81 
Jason Elam  36.85  200/214  93/130  26/42  0.83  34  0.81  0.81 
Nick Novak  36.81  81/92  31/41  12/20  0.81  47  0.81  0.81 
John Carney  35.25  213/236  77/108  11/18  0.83  31  0.81  0.81 
Mike Nugent  36.42  132/145  57/76  10/24  0.81  43  0.81  0.81 
Alex Henery  36.07  52/57  22/29  3/8  0.82  40  0.81  0.81 
Matt Prater  37.92  107/115  43/66  25/35  0.81  48  0.81  0.81 
Cairo Santos  35.10  17/18  7/10  1/2  0.83  27.5  0.81  0.81 
David Akers  36.34  292/318  110/159  27/52  0.81  45  0.81  0.81 
Doug Brien  36.93  80/90  48/61  7/17  0.80  51  0.81  0.81 
Rian Lindell  36.09  209/226  75/111  23/42  0.81  49  0.80  0.81 
Al Del Greco  35.61  48/52  20/30  1/2  0.82  37  0.80  0.81 
Jeff Reed  34.94  169/183  54/79  8/17  0.83  33  0.80  0.81 
Graham Gano  37.27  77/89  40/54  12/20  0.79  59  0.80  0.81 
Caleb Sturgis  37.72  36/39  14/20  6/13  0.78  61.5  0.80  0.81 
Neil Rackers  37.10  183/203  68/95  26/53  0.79  60  0.80  0.80 
Jay Taylor  34.67  3/4  1/1  1/1  0.83  27.5  0.80  0.80 
Olindo Mare  35.54  238/267  89/117  19/41  0.81  42  0.80  0.80 
Morten Andersen  35.73  136/150  53/77  8/16  0.81  46  0.79  0.80 
David Buehler  38.15  13/16  8/11  4/6  0.76  72  0.79  0.80 
Mike Hollis  35.89  85/98  36/51  7/11  0.80  54.5  0.79  0.80 
Clint Stitser  30.75  6/7  1/1  0/0  0.88  6  0.78  0.80 
Paul Edinger  38.40  74/92  49/68  16/24  0.76  73  0.78  0.80 
Martin Gramatica  37.20  114/128  37/63  19/29  0.77  63  0.78  0.80 
Lawrence Tynes  34.80  153/173  42/60  11/21  0.81  44  0.78  0.80 
Norm Johnson  37.00  28/32  17/22  1/6  0.77  68  0.77  0.79 
Kris Brown  37.02  167/194  76/111  18/35  0.77  67  0.77  0.79 
Todd Peterson  35.56  113/131  43/59  8/17  0.79  58  0.77  0.79 
Pete Stoyanovich  36.21  36/40  15/26  3/4  0.77  64.5  0.77  0.79 
Billy Cundiff  36.39  136/156  52/73  9/29  0.76  69  0.76  0.79 
John Hall  36.99  128/144  51/84  10/23  0.75  74  0.76  0.79 
Brett Conway  36.30  49/59  20/27  5/11  0.76  70  0.76  0.78 
Bill Gramatica  35.69  25/27  10/15  2/6  0.77  66  0.76  0.78 
Cary Blanchard  37.44  29/34  13/21  3/6  0.74  78  0.75  0.78 
Steve Christie  35.25  105/121  35/59  10/17  0.76  71  0.74  0.78 
Dave Rayner  37.41  42/51  19/29  5/11  0.73  81  0.74  0.78 
Chris Jacke  37.27  25/28  6/11  1/5  0.73  80  0.74  0.77 
John Potter  34.75  2/2  1/1  0/1  0.75  75.5  0.73  0.77 
Todd France  33.78  5/5  2/4  0/0  0.78  61.5  0.73  0.77 
Brad Daluiso  33.76  40/46  13/23  1/1  0.77  64.5  0.72  0.77 
Craig Hentrich  48.50  2/2  2/3  1/5  0.50  104.5  0.72  0.77 
Cedric Oglesby  30.00  4/5  1/1  0/0  0.83  27.5  0.72  0.76 
Table 9
Kicker  avg attempt  <40  40  49  ≥50  pct  rank 


Richie Cunningham  35.03  35/43  12/18  1/4  0.74  77  0.71  0.76 
Jeff Chandler  34.60  16/18  6/10  0/2  0.73  79  0.70  0.76 
Wade Richey  35.36  58/69  19/35  3/7  0.72  82  0.70  0.75 
Tim Seder  35.95  30/36  14/23  0/3  0.71  84  0.70  0.75 
Aaron Elling  36.75  14/17  4/7  1/4  0.68  90  0.68  0.75 
Eddie Murray  35.65  12/14  4/8  0/1  0.70  86  0.68  0.75 
Doug Pelfrey  35.98  29/36  7/15  3/6  0.68  88  0.67  0.74 
Taylor Mehlhaff  32.75  2/3  1/1  0/0  0.75  75.5  0.67  0.74 
Michael Husted  35.93  32/44  14/21  1/4  0.68  89  0.66  0.74 
Brian Gowins  35.83  3/3  1/2  0/1  0.67  93.5  0.66  0.73 
Jose Cortez  34.07  41/50  11/24  1/1  0.71  85  0.65  0.73 
Steve Lindsey  32.71  3/5  2/2  0/0  0.71  83  0.65  0.73 
Greg Davis  36.68  11/15  6/11  1/2  0.64  97  0.64  0.73 
Jeff Jaeger  34.85  19/23  2/9  2/2  0.68  91  0.64  0.73 
Jon Hilbert  37.39  9/10  2/8  0/0  0.61  99  0.63  0.72 
Kris Heppner  35.00  8/11  2/2  0/2  0.67  93.5  0.63  0.72 
Justin Medlock  34.83  6/7  2/4  0/1  0.67  93.5  0.63  0.72 
Chris Boniol  35.30  19/24  6/14  1/2  0.65  96  0.63  0.72 
Brandon McManus  34.85  7/8  2/3  0/2  0.69  87  0.62  0.72 
Jake Arians  37.90  8/10  4/11  0/0  0.57  102  0.61  0.71 
Seth Marler  35.88  14/19  5/12  1/2  0.61  100  0.59  0.70 
James Tuthill  35.06  7/10  2/5  1/1  0.63  98  0.59  0.70 
Ola Kimrin  36.40  5/6  1/3  0/1  0.60  101  0.59  0.70 
Ricky Schmitt  33.33  2/3  0/0  0/0  0.67  93.5  0.59  0.70 
Owen Pochman  41.59  4/9  4/5  0/3  0.47  106  0.58  0.69 
Hayden Epstein  35.33  5/6  0/2  0/1  0.56  103  0.52  0.67 
Nate Freese  38.14  3/3  0/4  0/0  0.43  107  0.45  0.63 
Shane Andrus  38.20  2/3  0/2  0/0  0.40  108  0.45  0.63 
Michael Koenen  44.00  2/5  0/1  2/8  0.29  111  0.45  0.63 
Scott Blanton  39.33  1/2  0/1  0/0  0.33  109.5  0.42  0.62 
Aaron Pettrey  29.25  2/3  0/1  0/0  0.50  104.5  0.32  0.56 
Toby Gowin  34.67  1/2  0/1  0/0  0.33  109.5  0.30  0.56 
For veteran kickers with a large number of kicks that are similar in distribution to that of the whole league, the effect of shrinkage is minimal. Adam Vinatieri has succeeded on 84% of 572 attempted kicks at an average distance of 35.8. His modelbased estimate averaged over the distribution of all NFL kicks in 19982014 is 0.83 and the shrinkage estimate is only slightly different at 0.82. Conversely kickers like Patrick Murray and Greg Zuerlein have achieved very successful records so far in their young careers, hitting 20/24 (83%) and 73/89 (82%), at longerthanaverage average distances of 41.1 yards and 40.0 yards respectively. Their modelbased estimates averaged over the leaguewide distribution of distances is more favorable (87% and 86%; note the bump up in rank from that based on the empirical frequency) but the shrinkage estimate brings these closer to more average kickers (84% for both kickers).
These tables suggest a ranking that places many current kickers among the best over the last 16 year period. Justin Tucker has been successful on 105 out of 116 attempts, with average distance
4.2Stadium effects
A topic of interest for many fans, writers and announcers is the effects of certain stadiums on the chances of making a field goal. The open end of Heinz Field receives particular attention for increased difficulty due to wind (Batista, 2002). Modeling fixed factorial effects for stadiums enables investigation of this issue. Though a likelihood ratio test finds evidence of stadium effects (χ^{2} = 112, p < 0.0001, df = 50), their inclusion does not lead to an improved fit as assessed by HosmerLemeshow, where there is a greater discrepancy between observed and expected counts than the model based on kickers and distances alone (HL = 8.3with stadium effects and HL = 1.9 without stadium effects, from Table 3). Nevertheless, including these effects does help to identify the more difficult and more favorable stadiums in which to kick, though again the estimation may suffer from selectionbias.
Success probabilities for each stadium can be estimated by backtransforming the marginal mean on the CLL scale. These marginal means average equally over all kickers and correspond to the average distance of
Table 11
Franchise  Stadium  games 
 att/g  made/att  pct 
 Std.err. 
Colts  Lucas Oil Stadium  62  37.4  3.9  217/244  0.89  0.88  0.03 
Lions  Pontiac Silverdome  32  38.2  4.0  109/127  0.86  0.87  0.04 
Broncos  Sports Authority Field at Mile High  119  37.9  3.7  375/442  0.85  0.84  0.02 
Vikings  Mall of America Field  131  37.0  3.7  408/483  0.84  0.84  0.02 
Lions  Ford Field  107  38.1  3.8  344/406  0.85  0.83  0.02 
Giants/Jets  MetLife Stadium  82  36.4  3.8  270/313  0.86  0.83  0.03 
Ravens  M&T Bank Stadium  141  36.2  4.1  494/572  0.86  0.83  0.02 
Cowboys  Texas Stadium  90  37.0  3.8  267/342  0.78  0.82  0.03 
Eagles  Lincoln Financial Field  103  36.0  3.7  324/383  0.85  0.81  0.03 
Rams  The Dome at America’s Center  140  38.6  3.9  454/551  0.82  0.80  0.02 
Saints  MercedesBenz Superdome  134  36.6  3.7  410/492  0.83  0.80  0.02 
Cowboys  AT&T Stadium  51  37.6  3.8  167/196  0.85  0.79  0.04 
Texans  NRG Stadium  107  37.8  3.8  327/403  0.81  0.79  0.03 
Falcons  Georgia Dome  141  36.4  3.7  433/525  0.82  0.79  0.02 
Panthers  Bank of America Stadium  140  36.8  3.7  425/519  0.82  0.78  0.02 
Eagles  Veterans Stadium  44  36.5  3.8  131/167  0.78  0.78  0.04 
Seahawks  CenturyLink Field  114  36.6  3.8  361/433  0.83  0.78  0.03 
Cardinals  Sun Devil Stadium  64  38.0  3.6  173/231  0.75  0.78  0.03 
Chiefs  Arrowhead Stadium  138  36.7  3.9  435/543  0.80  0.77  0.02 
Chargers  Qualcomm Stadium  141  35.9  3.7  427/525  0.81  0.77  0.02 
Buccaneers  Raymond James Stadium  140  37.3  3.7  417/515  0.81  0.77  0.02 
Dolphins  Hard Rock Stadium  142  36.9  4.1  470/579  0.81  0.77  0.02 
Jaguars  EverBank Field  114  37.3  3.6  323/410  0.79  0.77  0.03 
Colts  RCA Dome  87  36.0  4.0  282/345  0.82  0.76  0.03 
49ers  Candlestick Park  132  35.8  3.8  403/500  0.81  0.76  0.02 
Bengals  Paul Brown Stadium  123  36.0  3.8  375/462  0.81  0.74  0.03 
Patriots  Gillette Stadium  120  35.3  3.7  364/441  0.83  0.74  0.03 
Giants/Jets  Giants Stadium  199  34.9  3.7  594/740  0.80  0.74  0.02 
Raiders  OaklandAlameda County Coliseum  140  37.8  4.1  441/573  0.77  0.74  0.02 
Titans  Nissan Stadium  132  36.8  3.8  400/498  0.80  0.74  0.03 
Bills  New Era Field  129  35.3  3.9  403/504  0.80  0.74  0.03 
Packers  Lambeau Field  146  36.4  3.8  431/548  0.79  0.73  0.03 
Cardinals  University of Phoenix Stadium  77  36.8  4.0  248/311  0.80  0.73  0.03 
Patriots  Foxboro Stadium  33  35.6  4.3  111/143  0.78  0.73  0.04 
Steelers  Heinz Field  123  35.9  3.6  350/445  0.79  0.72  0.03 
Browns  FirstEnergy Stadium  128  35.1  3.8  388/482  0.80  0.71  0.03 
Redskins  FedExField  138  36.3  3.7  390/514  0.76  0.71  0.02 
Bears  Soldier Field  134  35.9  3.7  382/495  0.77  0.69  0.03 
The ranking in Table 11 seems to corroborate the anecdote that Heinz Field is a tough place to kick, as does the lowerthanaverage distance of field goal attempts. The new stadium in Denver, “Sports Authority Field at Mile High,” also has a reputation as a good place to kick and it does turn up high on this list. The estimates based on marginal means also tend to be lower than the empirical success rates. This observation may possibly be due to the fact that marginal means average equally over all kickers. Empirical success rates will tend to involve more attempts from highfrequency kickers, who tend to be better kickers.
4.3Big legs
The capacity to kick longdistance field goals is of particular interest to many football fans as well as NFL team management. One way to enrich the model to allow rankings among kickers to vary across distance is to expand the degree of the polynomial to quadratic. Using L and Q superscripts for the linear and quadratic kicker effects leads to the following representations of the generalized linearmodel:
For this investigation, attention is restricted to the K′ = 54 veteran kickers with at least 100 attempts in their careers, since small sample sizes present greater separability issues for fitting quadratic models. A comparison of the model with kickerspecific slopes nested within the quadratic model is not significant using this restricted dataset (χ^{2} = 63.3, df = 54, p = 0.181). This nonsignificance could possibly be explained by selection bias, as kickers with many attempts will all tend to be good enough to remain in the league. While not significant, there is a preponderance of kickers for whom the estimated quadratic coefficients have small pvalues, suggesting that there are some kickers for whom quadratic coefficients are nonzero. In fact the average pvalue for these 54 tests of the form
Table 10 enables simultaneous inspection of kickers who have estimated success probabilities that rank highly (top 20) in at least one of the three distances, 35, 45 or 55 yards. Entries in the table are sorted by the success probability estimate at 55 yards,
Fig.6
Table 10
career  ranks 
 empirical success  
Kicker  span  d = 35  45  55  35  45  55  30  39  40  49  50  59 
Garrett Hartley  0814  46  11  1  0.85  0.75  0.62  27/35  28/34  6/8 
Dan Bailey  1114  3  2  2  0.91  0.80  0.60  38/40  35/39  17/25 
Justin Tucker  1214  1  1  3  0.92  0.81  0.60  32/33  26/30  14/19 
Paul Edinger  0005  53  38  4  0.82  0.72  0.59  35/51  49/68  16/24 
Stephen Gostkowski  0614  22  8  5  0.88  0.76  0.59  94/105  65/85  14/18 
Blair Walsh  1214  9  3  6  0.89  0.78  0.58  30/33  19/24  17/23 
Connor Barth  0814  7  4  7  0.90  0.77  0.57  33/37  42/51  13/20 
Rob Bironas  0513  15  6  8  0.88  0.76  0.57  75/83  71/92  23/31 
Phil Dawson  9914  31  15  9  0.87  0.75  0.56  113/130  85/115  35/48 
Matt Bryant  0214  19  13  10  0.88  0.75  0.56  101/110  73/95  19/33 
Mike Hollis  9802  49  37  11  0.84  0.72  0.55  43/51  36/51  7/11 
Matt Prater  0714  33  19  12  0.87  0.74  0.55  52/57  43/66  24/34 
Steven Hauschka  0814  8  7  13  0.90  0.76  0.54  52/55  40/49  9/15 
Nick Folk  0714  41  29  14  0.86  0.73  0.54  66/74  57/78  19/32 
Jason Hanson  9812  10  9  15  0.89  0.76  0.54  107/118  108/141  43/67 
Graham Gano  0914  43  32  16  0.85  0.72  0.54  31/40  40/54  12/19 
Adam Vinatieri  9814  26  18  17  0.88  0.74  0.54  150/178  135/173  26/45 
Josh Brown  0314  13  14  18  0.89  0.75  0.54  91/100  81/111  35/54 
Shaun Suisham  0514  32  22  19  0.87  0.74  0.53  66/78  76/94  6/18 
Jay Feely  0114  40  30  20  0.86  0.73  0.53  121/141  95/130  18/32 
Dan Carpenter  0814  4  5  21  0.90  0.77  0.53  52/55  65/82  19/32 
Jeff Wilkins  9807  23  20  22  0.88  0.74  0.52  74/82  67/100  28/40 
Mike Vanderjagt  9806  12  16  24  0.89  0.75  0.52  69/76  71/95  15/22 
Sebastian Janikowski  0014  14  17  27  0.88  0.74  0.52  116/129  111/148  46/79 
Jason Elam  9809  18  21  30  0.88  0.74  0.51  94/106  93/130  25/39 
Robbie Gould  0514  5  10  33  0.90  0.75  0.51  81/90  74/100  17/22 
Ryan Longwell  9811  20  27  34  0.88  0.73  0.51  120/135  97/133  24/40 
Shayne Graham  0114  16  23  36  0.88  0.74  0.50  97/106  69/89  15/29 
Joe Nedney  9810  11  28  44  0.89  0.73  0.48  69/77  62/88  17/31 
Nate Kaeding  0412  17  31  45  0.88  0.72  0.48  50/54  51/73  10/19 
John Kasay  9811  2  12  47  0.91  0.75  0.48  82/87  88/114  32/60 
Stover  9809  6  33  52  0.90  0.72  0.44  101/111  97/131  9/20 
Table 13 helps quantify the variancereducing property of shrinkage estimation by comparison of the coefficient of variation among kickers with maximum likelihood (no shrinkage) estimates. The table gives results for both linear and quadratic fits, with diminished coefficient of variation for shrinkage estimates, particularly for shorter attempts.
Table 13
Statistic  Model  Method  40  45  50  55  60  65 
Mean  Linear  ML  0.80  0.71  0.62  0.54  0.45  0.38 
shrinkage  0.81  0.73  0.64  0.52  0.32  0.19  
Quadratic  ML  0.81  0.72  0.62  0.52  0.43  0.34  
shrinkage  0.81  0.74  0.64  0.51  0.30  0.17  
Coef.  Linear  ML  5.93  8.25  10.77  13.41  16.13  18.87 
Var.  Shrinkage  2.92  4.01  5.26  6.90  11.50  18.87  
Quadratic  ML  6.36  7.94  10.60  15.31  22.69  33.11  
Shrinkage  3.15  3.88  5.16  7.77  15.89  33.11 
Another way to rank kickers is to consider biglegs and stadium effects and shrinkage estimation all together. Stadium effects can bias a ranking and by including them in the model, together with quadratic distance effects, one can attempt to rank kickers on an even footing. The model is then given by
Table 12
career  ranks 
 empirical success  
Kicker  span  d = 35  45  55  35  45  55  30  39  40  49  50  59 
Garrett Hartley  0814  48  15  1  0.85  0.75  0.62  27/35  28/34  6/8 
Dan Bailey  1114  4  2  2  0.91  0.80  0.61  38/40  35/39  17/25 
Stephen Gostkowski  0614  23  8  3  0.88  0.77  0.59  94/105  65/85  14/18 
Paul Edinger  0005  53  33  4  0.83  0.73  0.59  35/51  49/68  16/24 
Justin Tucker  1214  1  1  5  0.92  0.81  0.58  32/33  26/30  14/19 
Connor Barth  0814  6  4  6  0.90  0.78  0.57  33/37  42/51  13/20 
Rob Bironas  0513  12  6  7  0.89  0.77  0.57  75/83  71/92  23/31 
Blair Walsh  1214  8  7  8  0.90  0.77  0.57  30/33  19/24  17/23 
Mike Hollis  9802  41  21  9  0.86  0.74  0.56  43/51  36/51  7/11 
Matt Bryant  0214  25  11  10  0.88  0.76  0.56  101/110  73/95  19/33 
Phil Dawson  9914  16  10  11  0.89  0.76  0.55  113/130  85/115  35/48 
Graham Gano  0914  43  31  12  0.86  0.73  0.55  31/40  40/54  12/19 
Shaun Suisham  0514  26  16  13  0.88  0.75  0.54  66/78  76/94  6/18 
Steven Hauschka  0814  7  9  14  0.90  0.76  0.53  52/55  40/49  9/15 
Dan Carpenter  0814  5  5  15  0.91  0.77  0.53  52/55  65/82  19/32 
Jay Feely  0114  36  25  16  0.87  0.74  0.53  121/141  95/130  18/32 
Josh Brown  0314  21  17  17  0.88  0.75  0.53  91/100  81/111  35/54 
Nick Folk  0714  45  38  18  0.86  0.72  0.53  66/74  57/78  19/32 
Adam Vinatieri  9814  31  23  19  0.88  0.74  0.53  150/178  135/173  26/45 
Robbie Gould  0514  2  3  20  0.91  0.78  0.52  81/90  74/100  17/22 
Sebastian Janikowski  0014  11  13  21  0.89  0.76  0.52  116/129  111/148  46/79 
Jeff Reed  0210  17  18  24  0.88  0.74  0.52  86/94  54/79  8/16 
Jason Hanson  9812  22  20  25  0.88  0.74  0.52  107/118  108/141  43/67 
Mike Vanderjagt  9806  9  14  27  0.90  0.75  0.52  69/76  71/95  15/22 
Shayne Graham  0114  14  19  34  0.89  0.74  0.50  97/106  69/89  15/29 
Ryan Longwell  9811  20  30  37  0.88  0.73  0.50  120/135  97/133  24/40 
Mike Nugent  0514  19  32  40  0.88  0.73  0.49  65/77  57/76  10/24 
Mason Crosby  0714  15  28  41  0.89  0.73  0.49  77/87  56/76  23/49 
Joe Nedney  9810  10  22  43  0.90  0.74  0.49  69/77  62/88  17/31 
Nate Kaeding  0412  18  36  46  0.88  0.73  0.48  50/54  51/73  10/19 
John Kasay  9811  3  12  48  0.91  0.76  0.47  82/87  88/114  32/60 
Matt Stover  9809  13  45  54  0.89  0.71  0.43  101/111  97/131  9/2 
4.4Nonstationarity
The analyses in this paper cover a wide time span. Kicker ability appears to be changing, especially in the last 10 years, with improving accuracy and experience among kickers (Clark et al., 2013). The frequencies of long field goal attempts are also increasing. Figure 9 shows the log of attempt frequencies against year, with separate plotting characters and colors for each distance, rounded to the nearest five yards. The logscale is better for visualizing the increase in frequencies. In fact, the log of the frequency of kicks between 53 and 57 yards, denoted in the graph as “55”, and also of kicks between 48 and 52 yards, denoted as “50”, appear to be linearly increasing with season. The figure overlays lines using estimated coefficients from Poisson regressions of frequency on year, separate for each 5yard distance interval, with the natural loglink function for the generalized linear model. The slopes are significant for distances of 50, 55, and 60 yards.
Fig.9
5Discussion
With the goal of variancereduction akin to that achieved by Efron and Morris (1975) and Brown (2008) in the prediction of endofseason batting averages, we presented a flexible framework for the construction of shrinkage estimators of field goal success probabilities. This framework allows for modelbased kickerspecific maximum likelihood estimates to be balanced with modelfree empirical frequencies aggregated over all kickers. For the modelbased estimator, various link functions were assessed using maximum likelihood, HosmerLemeshow statistics and restrictions on data motivated by possible selection bias. The complementary loglog function was selected by all criteria. Findings from several choices for the weight functions of the modelbased estimate were presented, including constant, functional, or distributionbased weights, but our investigation was by no means exhaustive. We recommend the midpoint estimator which weights the two components equally on the basis of simplicity and generally good performance as measured by the HosmerLemeshow statistic under cross validation. Interested readers are invited to give other weighting schemes a try. In particular, it seems that lowfrequency kickers ought to be estimated using more shrinkage toward the empirical frequency. Since these kickers constitute a comparatively small fraction of the data set, they do not carry much weight as we assess the performance of shrinkage estimators. It is reasonable that in some personnel decisions, they should carry more weight, and a different shrinkage approach and method of assessment may be more appropriate.
Another limitation of the study has to do with failed attempts that are not the fault of the kicker, or perhaps partially the fault of the kicker. If a snap or hold is bad, but an attempt is made and missed, it may appear in a verbal recap, but appears in the playbyplay simply as “no good.” Our model does not account for this type of event. Excluding blocked kicks is also problematic as it is possible that longer attempts may be more likely to be blocked by lesser kickers, so that these failed attempts contain information about kicker ability.
Using the approach of Berry and Berry (1985) to average success probability estimates over the observed distribution of attempt distances, rankings for kickers were provided. Interestingly several current kickers came out on top. In addition to this overall ranking, distance was fixed at several lengths, d = 35, 45 and 55 yards, corresponding roughly to estimated success probabilities for elite (top 20) kickers in the range of 0.9, 0.8, and 0.6 respectively. These estimates are based on a version of the model that expands the degree of polynomial dependence on distance to be quadratic. Presentation of the bestkickers sidebyside enables investigation of kickers who have “big legs,” with a decent shot at long kicks, in comparison to those who are “automatic” at medium distances. An attempt to refine these rankings was made by adding stadium effects, which were found to be highly significant. The estimated marginal (averaged over kickers) means for stadiums with adequate sample sizes were backtransformed to give a ranking of the toughest venues in which to kick.
General managers have made kicker personnel changes with surprising frequency, perhaps overreacting after the sting of a loss which involved a missed field goal. Let us consider the case of Garrett Hartley in more detail. Hartley began his career with the New Orleans Saints in 2008 by making all 13 of his field goal attempts. The following year he made all but two field goal attempts, with the misses coming from 58 and 37 yards. As mentioned in Section 1, in 2010 Hartley missed a 29 yard attempt in overtime in a week 4 loss and the Saints chose to sign John Carney. Carney proceeded to make 5 out of 6 field goals for the two games he replaced Hartley, missing from 29 yards in week 5, the exact same yardage which precipitated Hartley’s demotion. So the following game, Carney was demoted in favor of Hartley and Hartley proceeded to make 16 of the 18 remaining kicks he attempted in the season. The frequency of personnel changes among kickers, averaging more than 3 midseason cuts per year, is surprising in light of the small number of total kickers over the 16 year period of this study. Though kickers may lose their jobs, many find employment shortly after termination, with a small number moving on through free agency. Whether the high turnover is a result of misunderstanding of the sampling variability inherent in estimating success probabilities from a small number of binary trials, or due to reaction to fan hostility is unclear.
Over the last five years, there has been an increase in the distances at which field goals are attempted, most likely due to increases in kicker abilities. Attempting to model this increase could lead to improvements in the estimation of success probabilities and would be an excellent avenue for further study. Another potentially interesting topic that could be investigated would be the effect of age on elite kickers who have exhibited longevity in the league. Here it is implicitly assumed to be constant, but we may wonder if it is reasonable to expect the same degree of accuracy at all stages of a kicker’s career.
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