NFL Y2K PCA

Mitchell Watnik and Richard A. Levine
University of California

Journal of Statistics Education Volume 9, Number 3 (2001)

This text may be freely shared among individuals, but it may not be republished in any medium without express written consent from the authors and advance notification of the editor.

Key Words: Multivariate analysis; National Football League (NFL); Summary ranking measures.

Abstract

The dataset associated with this paper is from the 2000 regular season of the National Football League (NFL). We use principal components techniques to evaluate team "strength." In some of our analyses, the first two principal components can be interpreted as measure of "offensive" and "defensive" strengths, respectively. In other circumstances, the first principal component compares a team against its opponents.

1. Introduction

Our dataset is from the National Football League (NFL), but our work did not begin that way. We were interested in discussing the football team at our workplace, the University of California at Davis. The Aggies are an excellent football team, regularly win their league, and have been nationally ranked within NCAA Division II for a few years. In the 2000 season, the team made it to the semifinal game of the Division II championship series.

Just prior to that game, a local newspaper indicated that the Aggies' much-maligned defense was actually ranked in the top 20 in Division II. We did not believe that to be true. While we may be fair-weather fans of the Aggies, we felt that they were somewhat comparable to the 2000 version of the NFL St. Louis Rams -- all offense, no defense.

We decided to come up with a better way of ranking defenses, even going so far as to name the article before it was written ("The Best Defense is a Great Offense? Taking the Quarterback Out of Defense Rankings"). Our idea was that the amount of time that the defense is on the field is not typically accounted for in ranking defenses. A defense that is typically on the field for 25 minutes per 60-minute game is probably going to give up fewer points than a defense that is on the field for 30 minutes per game. Our first thought was to compute rates over time for touchdowns, yardage gained, and other accumulated statistics. A ranking of offenses could effectively use the same scoring technique as that for defenses (where a high score indicates a poor defense). We note that rates have been used previously for improving summary measures in sports statistics. For example, Anderson-Cook, Thornton, and Robles (1997) suggest a beautiful use of rates for evaluating power-play efficiency in hockey.

The first issue, of course, was to acquire data, but getting what we felt to be "necessary" information about Division II football teams is a formidable task. We therefore set out to rank NFL teams since the data are much more readily available.

The next problem is obvious: there is a lot of statistical information available for the taking. What is important and what is not important in ranking offenses and defenses is anyone's guess. Nonetheless, just as is the case with the title of this paper, a great deal of information can be effectively summarized using well-known dimension reduction techniques. We therefore employed the usual statistical methodology for when one has numerous variables, but a relatively small number of observations -- principal components (see Johnson and Wichern 1998 for an introduction). As noted in some detail in the sequel, the technique worked in almost textbook fashion.

2. The Dataset

These data consist of information from the 2000 regular season (not including playoffs) of the NFL. Most of the information was obtained via the NFL web site www.nfl.com, though some, particularly the information pertaining to starting field position, was obtained from www.foxsports.com. Any "rate" variable has the average time of possession times the number of games as the denominator. The variables in the dataset are the number of touchdowns (touch), total offensive yards (yards), time of possession (top), rate of touchdowns (ratetd), number of sacks (sacks), rate of yards (rateyds), number of drives beginning in the "red zone" (drives20), number of drives beginning in "opponents' territory" (drives50), field goals attempted (fga), field goals made (fgm), number of punts (puntno), gross punt average (puntave), net punt average (puntnet), number of punts going for touchbacks (punttb), number of punts placed within the 20 yard line (punt20), longest punt return (puntlong), punt rate (puntrate), number of punts blocked (puntblock), number of first downs (1sts), number of kickoffs (kos), amount of return yardage on the kickoff (koyds), average length of kickoff returns (koave), number of kickoffs returned for a touchdown (kotds), number of punts returned (rets), number of punts "fair caught" (fc), amount of punt return yardage (retyds), average length of punt returns (retave), number of punts returned for a touchdown (rettds), number of interceptions (int), and number of fumble recoveries (recover). Each of these pieces of information applies to both the team of interest and their opponents -- the former will be prefixed by "home" and the latter will be prefixed by "opp." We also have each team's wins and losses.

3. Our Initial Analysis

Although we compiled this dataset, we have no doubt that ours will not be the final word on its analysis. Indeed, our hope is that students will come up with novel and statistically sound ways of summarizing and analyzing this NFL data.

We used SAS® "proc princomp" to perform the principal components analysis on the raw explanatory information, and, as will be seen, we tried various configurations of variables.

Our first attempt at the analysis involved only a few variables, because, at the time, these variables were the only ones available. Furthermore, we had information only for the American Football Conference (AFC; about half of the teams in the NFL). The first two principal components, given in Table 1 explain almost 82% of the variation. The corresponding biplot (see, for example, Section 12.7 of Johnson and Wichern 1998 or Venables and Ripley 1997, pp. 388-389) from S-Plus® is given in Figure 1, where the abbreviations for the variables used in the figure are given in Table 1 and those for the teams are given in Table 2; teams that made the playoffs are indicated by asterisks in the tables and figures. In order to avoid some confusion, we note that on Figure 1, there is a counterintuitive correspondence between the points and the graph labels. The lighter axes (those on the upper and right-hand parts of the plot) correspond to the darker points (the team names), and vice-versa (see Venables and Ripley 1997, p. 388).

Table 1. First Two Principal Components Using the American Football Conference (AFC) Data.

 Variable First PC Second PC hometop (htop) 0.289154 -0.268395 hometouch (htd) 0.393211 0.103344 opptouch (otd) -0.099851 0.417011 homeyards (hyd) 0.395883 0.054983 oppyards (oyd) -0.056191 0.410637 homeratetd (hrtd) 0.362950 0.177276 oppratetd (ortd) -0.002532 0.409282 homerateyds (hryds) 0.302423 0.233419 opprateyds (oryds) 0.190472 0.320558 oppdrives20 (odriv20) -0.239607 0.231254 oppdrives50 (odriv50) -0.321779 0.138639 home1sts (h1sts) 0.397584 0.070433 opp1sts (o1sts) -0.115843 0.371349

Table 2. Team Rankings Based on the Difference in the First Two Principal Components fromTable 1 for All Teams in the National Football League. Asterisks (*) denote teams that made the NFL 2000 playoffs.

 Overall Rank Team Conf. Wins Overall Score Offensive Score Off. Rank Defensive Score Def. Rank 1 TEN * AFC 13 5.38075 1.26479 8 -4.11596 2 2 BAL * AFC 12 4.66794 0.31373 12 -4.35421 1 3 WAS NFC 8 2.14909 0.25058 15 -1.89851 5 4 OAK * AFC 12 1.86825 2.92732 3 1.05907 21 5 MIA * AFC 11 1.84852 -0.67641 21 -2.52493 3 6 NYG * NFC 12 1.75330 0.67552 11 -1.07779 7 7 DEN * AFC 11 1.55059 4.16786 2 2.61728 28 8 PIT AFC 9 1.51409 -0.42227 19 -1.93636 4 9 NO * NFC 10 1.23657 1.05029 10 -0.18628 12 10 PHI * NFC 11 1.19001 0.02921 17 -1.16080 6 11 IND * AFC 10 1.18673 2.72832 4 1.54159 23 12 BUF AFC 8 1.02534 0.32804 13 -0.69730 10 13 JAX AFC 7 0.89662 1.07693 9 0.18031 13 14 NYJ AFC 9 0.86557 0.23653 16 -0.62903 11 15 TB * NFC 10 0.60569 -0.29121 18 -0.89690 9 16 STL * NFC 10 0.27546 4.78642 1 4.51096 31 17 GB NFC 9 -0.03321 0.31085 14 0.34406 15 18 DET NFC 9 -0.22774 -1.15149 23 -0.92375 8 19 KC AFC 7 -0.37763 1.31450 7 1.69212 25 20 MIN * NFC 11 -0.54273 1.94430 6 2.48703 27 21 SF NFC 6 -0.88587 2.33624 5 3.22212 30 22 CAR NFC 7 -1.15463 -0.59112 20 0.56351 17 23 NE AFC 5 -1.78054 -1.29337 24 0.48717 16 24 DAL NFC 5 -2.11107 -1.46166 25 0.64941 18 25 CHI NFC 5 -2.59514 -2.26068 27 0.33447 14 26 ATL NFC 4 -2.98308 -2.10208 26 0.88099 20 27 SEA AFC 6 -3.74492 -0.68732 22 3.05760 29 28 CIN AFC 4 -4.15162 -3.33577 30 0.81585 19 29 SD AFC 1 -4.65210 -3.03908 29 1.61302 24 30 ARI NFC 3 -4.84552 -2.47209 28 2.37343 26 31 CLE AFC 3 -6.09758 -4.90381 31 1.19377 22

Figure 1.

Figure 1. Biplot of PC Values inTable 1.

We interpret the first principal component as an "offensive score," summarizing a team's offensive capabilities. The second principal component may be interpreted as a "defensive score," summarizing a team's defensive capabilities. In the case of the first principal component, a large positive score indicates a good offensive team (indicated by being further to the right in Figure 1); in the second, a large negative score indicates a good defensive team (indicated by being closer to the bottom in Figure 1). We regressed these two principal components on team win percentage; the marginal regressions are depicted in Figures 2a and 2b. The R2 was 83%. We also found that the regression coefficients were close to equal, though of opposite signs. In fact, a hypothesis test -- see, for example, Samaniego and Watnik (1997) -- established that the difference between the two principal components, labeled "overall score" in Table 2 and Figure 2c, showed no significant difference between the model with just the overall score and the two separate scores.

Figure 2a.

Figure 2a. Plot of Offensive Scores Against Wins.

Figure 2b.

Figure 2b. Plot of Defensive Scores Against Wins.

Figure 2c.

Figure 2c. Plot of Overall Scores Against Wins.

Table 2 presents the offensive, defensive, and overall scores (as defined in the previous paragraph), using the first two principal components of Table 1, for every NFL team. The National Football Conference (NFC) teams in Table 2 provide a kind of "validation" of the AFC model. Six of the top seven AFC teams, according to this scoring criterion, made the playoffs. While the best NFC team according to this measure, the Washington Redskins, did not make the playoffs, the next five NFC teams did. (This could be taken as an indication that the team's win-loss record was not up to the teams' performance and thus an explanation for the Redskins' late-season firing of their coach.) As the reader might notice throughout, the Minnesota Vikings fared poorly in almost every model while the Washington Redskins tended to be overrated by the models. Interestingly, though not surprisingly, the St. Louis Rams had the NFL's best offense and the worst defense according to our model. Three playoff-qualifying AFC teams, the Oakland Raiders, Denver Broncos, and Indianapolis Colts, had a similar, but not as dramatic, imbalance.

Our attempt at principal components for the above variables using all of the NFL teams was a success. However, lest students think that principal component analyses on any subset would work, our attempt using just the NFC teams was not successful. That is, the principal component analysis of NFC data was not amenable to the clear offensive and defensive interpretation as the analysis of the AFC data. We were obviously fortunate to have chosen the AFC as the conference whose data would be entered first.

We were also fortunate to have the principal components come out in such a desirable (for us) way. As the reader will see shortly, when all (or most) of the variables are included in the analysis, the principal components method tends to look directly at the difference between the "home" and "opp" measures. This leads us to believe that the imbalance in the variables in this model is what caused these interpretable components.

4. Analysis of the Full Dataset

Of course, principal components can handle a much larger number of variables. There is no reason for us not to use every variable at our disposal. For the AFC only, the first principal component, contained in Table 3, explained only 29% of the variation. Nonetheless, this principal component, in our opinion, is a direct measurement of the team against its opponents. Namely, this principal component almost always subtracts the contribution of the opposing team from the corresponding contribution of its team for the offense and vice-versa for the defense.

Table 3. First Principal Components for the AFC Data Using All Variables.

 Variable First PC Variable First PC hometop 0.218719 hometouch 0.134745 opptouch -0.185692 homeyards 0.136346 oppyards -0.149335 homeratetd 0.095218 oppratetd -0.141504 homerateyds 0.029558 opprateyds -0.006153 home1sts 0.139670 opp1sts -0.163813 homesacks 0.093923 oppsacks -0.092164 homeint 0.133793 oppint -0.084340 homerecover 0.057836 opprecover 0.014744 homekos -0.227016 oppkos 0.193279 homekoyds -0.176723 oppkoyds 0.154004 homekoave 0.137286 oppkoave -0.110405 homekotds -0.014834 oppkotds -0.017435 homedrives20 0.148039 oppdrives20 -0.175901 homedrives50 0.168546 oppdrives50 -0.197861 homefga 0.199397 oppfga -0.203328 homefgm 0.203278 oppfgm -0.197710 homepuntno -0.135343 opppuntno 0.110739 homepuntrate -0.190366 opppuntrate 0.173589 homepuntave -0.077663 opppuntave 0.101796 homepuntnet 0.021132 opppuntnet -0.051215 homepunt20 0.113390 opppunt20 -0.020490 homerettds 0.115540 opprettds -0.031943 homeretyds 0.165704 oppretyds -0.158089 homefc -0.024124 oppfc 0.084470 homerets 0.127991 opprets -0.162967 homepunttb 0.025041 opppunttb 0.054527 homeretave 0.124335 oppretave -0.082901 homepuntlong 0.003435 opppuntlong -0.051890 homepuntblock 0.024799 opppuntblock 0.051452

Using just that principal component, the regression on winning percentage for AFC teams provided an R2 of 72%. In Table 4 and Figure 3, we show how the principal components matched with the teams' number of wins. Again, we used the NFC as a "validation" group. The top five AFC teams, according to this criterion, made the playoffs. Five of the top six NFC teams made the playoffs.

Table 4. Team Rankings Based on the First Principal Component from Table 3 for All Teams in the NFL. Asterisks (*) Denote Teams that Made the NFL 2000 Playoffs.

 Rank Team Conference First PC score Wins 1 BAL * AFC 6.92874 12 2 TEN * AFC 6.62203 13 3 OAK * AFC 3.19638 12 4 DEN * AFC 3.05194 11 5 MIA * AFC 2.44269 11 6 GB NFC 1.56921 9 7 STL * NFC 1.50917 10 8 PIT AFC 1.38003 9 9 JAX AFC 1.30208 7 10 NYG * NFC 1.26682 12 11 TB * NFC 1.14953 10 12 IND * AFC 0.96163 10 13 PHI * NFC 0.49389 11 14 NO * NFC 0.40795 10 15 WAS NFC 0.22149 8 16 DET NFC -0.05276 9 17 CAR NFC -0.36193 7 18 NE AFC -0.47124 5 19 BUF AFC -1.19575 8 20 KC AFC -1.35185 7 21 NYJ AFC -1.48123 9 22 SF NFC -1.48244 6 23 DAL NFC -1.65470 5 24 MIN * NFC -1.69589 11 25 CHI NFC -2.40573 5 26 ATL NFC -2.58762 4 27 SEA AFC -2.78394 6 28 CIN AFC -5.20318 4 29 SD AFC -5.26384 1 30 ARI NFC -5.41074 3 31 CLE AFC -8.13448 3

Figure 3.

Figure 3. Principal Component Scores Against Wins.

The first principal component, given in Table 5 for the entire dataset (including all of the variables and all of the teams) explained only 21% of the variation. Again, as in Table 3, it appears to compare the team to its opponents directly.

Table 5.First Principal Components Using All Variables and All NFL Teams.

 Variable First PC Variable First PC hometop 0.255711 hometouch 0.135058 opptouch -0.201184 homeyards 0.131536 oppyards -0.196629 homeratetd 0.095602 oppratetd -0.153265 homerateyds 0.024881 opprateyds -0.043073 home1sts 0.146149 opp1sts -0.205652 homesacks 0.136492 oppsacks -0.073768 homeint 0.169915 oppint -0.106793 homerecover 0.072983 opprecover 0.003678 homekos -0.237560 oppkos 0.215731 homekoyds -0.192624 oppkoyds 0.189581 homekoave 0.058840 oppkoave -0.009941 homekotds -0.043506 oppkotds -0.014173 homedrives20 0.151142 oppdrives20 -0.176760 homedrives50 0.188694 oppdrives50 -0.176526 homeFGa 0.135393 oppfga -0.201820 homeFGM 0.194181 oppfgm -0.181222 homepuntno -0.051007 opppuntno 0.153017 homepuntrate -0.130411 opppuntrate 0.212593 homepuntave -0.058642 opppuntave 0.104526 homepuntnet -0.015789 opppuntnet -0.021425 homepunt20 0.120944 opppunt20 0.029792 homerettds 0.082453 opprettds -0.018587 homeretyds 0.178650 oppretyds -0.165508 homefc 0.013974 oppfc 0.016949 homerets 0.142097 opprets -0.094260 homepunttb 0.054588 opppunttb 0.051486 homeretave 0.114561 oppretave -0.015596 homepuntlong 0.043231 opppuntlong -0.007458 homepuntblock 0.011052 opppuntblock 0.065411

Table 6. Team Rankings Based on the First Principal Component from Table 5 for All Teams in the NFL. Asterisks (*) Denote Teams that Made the NFL 2000 Playoffs.

 Rank Team Conference First PC score Wins 1 BAL * AFC 8.04573 12 2 TEN * AFC 7.50834 13 3 DEN * AFC 3.54837 11 4 OAK * AFC 3.43575 12 5 MIA * AFC 2.91691 11 6 NYG * NFC 2.34373 12 7 PIT AFC 2.31205 9 8 JAX AFC 2.28250 7 9 TB * NFC 1.87205 10 10 GB NFC 1.66451 9 11 NO * NFC 1.38613 10 12 IND * AFC 1.26829 10 13 STL * NFC 1.05026 10 14 PHI * NFC 0.86671 11 15 WAS NFC 0.72484 8 16 DET NFC 0.70963 9 17 BUF AFC 0.02548 8 18 NE AFC -0.33104 5 19 NYJ AFC -0.64109 9 20 KC AFC -0.79622 7 21 CAR NFC -1.31142 7 22 SF NFC -1.73040 6 23 MIN * NFC -1.80792 11 24 DAL NFC -2.08162 5 25 CHI NFC -2.39768 5 26 ATL NFC -3.04457 4 27 SEA AFC -3.81363 6 28 SD AFC -4.52281 1 29 CIN AFC -5.08288 4 30 ARI NFC -6.86933 3 31 CLE AFC -7.53067 3

The R2 for the regression of this principal component on the number of wins, as represented in Figure 4, was 73%. Here, the top five AFC teams and five of the top six NFC teams made the playoffs. Furthermore, the principal component correctly selected the Super Bowl opponents and outcome, as well as all of the AFC playoff outcomes. (Its performance with respect to the NFC playoff match-ups was only successful half the time -- the New York Giants' victories over the Philadelphia Eagles and the Minnesota Vikings and the New Orleans Saints' victory over the St. Louis Rams.)

Figure 4.

Figure 4. Principal Component Scores Against Wins.

Finally, we summarize the data separately by offensive, defensive, and special teams variables. Tables 7, 8, and 9 present the relevant principal components. The first principal component for the offense explains 46% of the variation and the first principal component for the defense explains 49% of the variation. Note that the offensive principal component has very similar coefficients to the defensive principal component, with the obvious exception of time of possession. We feel that the difference between these two principal component scores gives an indication of overall team strength. In our attempt to summarize the special teams data, we found that considering the punting and kicking teams separately was superior to trying to do them both at once. Furthermore, we found that the ability of the punting team only had a significant effect on the number of wins. Thus the variables used in Table 9 consist only of punting statistics. The first principal component in Table 9 seems to represent the return capabilities of a team, though it only explains 22% of the variation. The second and third principal components appear most interpretable, summarizing the abilities of the home punting team and the opposing punting team. They explain 16% and 13% of the variation, respectively. We utilize these latter two principal components to devise a punting score in evaluating the teams.

Table 7. First Principal Component Summarizing Offensive Variables for Data from All NFL Teams.

 Variable First PC hometouch 0.343352 homeyards 0.348787 hometop 0.232946 homeratetd 0.323398 oppsacks -0.132067 homerateyds 0.288800 homedrives20 0.053379 homedrives50 0.085324 homeFGa 0.119071 homeFGM 0.118911 home1sts 0.355936 home1rate 0.351317 homepuntno -0.291337 oppint -0.096390 opprecover -0.100766 homepuntrate -0.328446

Table 8. First Principal Component Summarizing Defensive Variables for Data from All NFL Teams.

 Variable First PC hometop -.294770 oppyards 0.326010 opptouch 0.303064 homesacks -.179711 opprateyds 0.179022 oppratetd 0.259483 oppdrives20 0.181012 oppdrives50 0.148804 oppfga 0.221289 oppfgm 0.202965 opp1sts 0.324007 opp1rate 0.332077 opppuntno -.265742 homeint -.195805 homerecover -.111704 opppuntrate -.318653

Table 9. First Three Principal Components Summarizing Punting Variables for Data from All NFL Teams.

 Variable First PC Second PC Third PC opppuntave 0.021507 0.006500 0.591860 opppuntnet 0.381688 -0.177946 0.290832 opppunttb -0.003666 -0.038939 0.162348 opppunt20 0.102521 -0.140235 -0.266048 opppuntblock 0.087499 0.222528 0.024679 homepuntave 0.216575 0.523767 -0.017731 homepuntnet -0.072828 0.519856 -0.101296 homepunttb 0.139907 0.355150 -0.181628 homepunt20 -0.173346 0.180112 0.240860 homepuntblock -0.229419 -0.116809 0.179845 homefc -0.046043 -0.054086 -0.522971 oppfc -0.209811 -0.313148 -0.110353 homeretave -0.441252 0.182222 0.149668 oppretave 0.431589 0.108961 0.122223 homerettds -0.377793 0.122072 -0.017254 opprettds 0.333716 -0.140810 -0.100679

The "total score" in Table 10 is computed as the offensive score (column 6 of the table and Figure 5a) minus the defensive score (column 7 and Figure 5b) plus one-half the punting score (column 8 and Figure 5c). These weights were suggested from the regression of win percentage on all of these three scores. This regression had an R2 of 81%. Not surprisingly, the R2 for the regression of this "total score" (see Figure 5d) on the number of wins was also 81%. Here, the top six NFC teams and the top five AFC teams made the playoffs. Interestingly, the Raiders and Jets punt teams push them up in the rankings. The Vikings, who have the best punting special team according to this analysis, also fair better. On the other hand, the Redskins are hurt by their punting team.

Table 10. Team rankings based on a summary of the offensive, defensive, and punting special teams play of each NFL team. The offensive and defensive principal components come from Tables 7 and 8. The punting summary measure consists of the difference of the second and third principal components from Table 9. The total score upon which the ranking is based consists of the offensive score minus the defensive score minus one-half the punting special teams score. Asterisks (*) denote teams that made the NFL 2000 playoffs.

 Rank Team League Wins Total score Off. score Def. score Punting score 1 TEN * AFC 13 8.1776 1.05733 -6.83163 0.57723 2 OAK * AFC 12 5.6500 3.64252 -0.80435 2.40616 3 BAL * AFC 12 5.4724 0.50198 -6.34898 -2.75720 4 DEN * AFC 11 4.5540 5.32779 -0.54343 -2.63447 5 STL * NFC 10 3.8212 6.30485 2.39239 -0.18245 6 NYG * NFC 12 3.3699 1.01238 -2.74760 -0.78009 7 IND * AFC 10 3.0062 3.26709 0.42423 0.32670 8 JAX AFC 7 2.9492 1.52267 -1.93469 -1.01628 9 NYJ AFC 9 2.8705 0.40729 -1.03838 2.84974 10 MIA * AFC 11 2.8688 -1.70385 -2.72417 3.69706 11 NO * NFC 10 3.0257 1.24803 -1.90010 -0.24487 12 PHI * NFC 11 2.1248 -0.30602 -1.01290 2.83591 13 TB * NFC 10 2.2483 -0.26549 -1.66600 1.69555 14 MIN * NFC 11 1.7890 2.29798 2.73073 4.44349 15 PIT AFC 9 1.5447 -0.48368 -1.87234 0.31219 16 GB NFC 9 1.4350 1.02025 -1.21816 -1.60686 17 WAS NFC 8 0.5425 0.31602 -1.80931 -3.16574 18 BUF AFC 8 -0.1661 -0.06044 -1.70203 -3.61544 19 DET NFC 9 -0.4335 -1.80718 -0.80075 1.14591 20 KC AFC 7 -0.8891 0.77321 1.13140 -1.06190 21 NE AFC 5 -2.1315 -1.65604 1.15651 1.36214 22 DAL NFC 5 -2.0687 -0.96984 1.92338 1.64898 23 SF NFC 6 -2.5398 2.53445 3.06935 -4.00972 24 CAR NFC 7 -3.4113 -0.25456 1.59191 -3.12961 25 CHI NFC 5 -3.6990 -3.40067 0.23609 -0.12443 26 SD AFC 1 -4.5464 -3.27735 2.14002 1.74198 27 ATL NFC 4 -4.8164 -2.61938 2.18382 -0.02650 28 SEA AFC 6 -6.0889 -1.15941 4.24062 -1.37769 29 CIN AFC 4 -6.6120 -4.14713 2.73793 0.54603 30 ARI NFC 3 -7.1163 -2.28034 4.88523 0.09852 31 CLE AFC 3 -10.9308 -6.84243 4.11121 0.04566

Figure 5a.

Figure 5a. Offensive PC Scores Against Wins.

Figure 5b.

Figure 5b. Defensive PC Scores Against Wins.

Figure 5c.

Figure 5c. Punting PC Scores Against Wins.

Figure 5d.

Figure 5d. Punting PC Scores Against Wins.

6. Other Uses

This dataset need not be limited to use in multivariate statistics courses. For example, one could discuss whether teams in the NFC score more touchdowns than teams in the AFC (and whether it is appropriate to use a two-sample t-test for these data). There are innumerable regression models that could be explored as well, but, as part of that, an interesting discussion could result from pointing out that the assumption of independence of observations is not met in this situation. Many students will recognize that the problem is not with, say homeint and oppint, being related (though there is collinearity), but with the number of wins across the teams that violates the assumption.

7. Conclusion

We have provided a reasonably comprehensive dataset for the 2000 NFL regular season. Furthermore, we presented and summarized some of our exploratory analyses on it. We believe that the dataset would be in a good example for use in multivariate statistics courses.

8. Getting the Data

The file nfl2000.dat.txt contains the raw data. The file nfl2000.txt is a documentation file containing a brief description of the dataset.

Appendix - Key To Variables in nfl2000.dat.txt

All rate variables use the total time of possession, that is the average time of possession times the number of games, as the denominator.

Each variable is provided for both the team of interest and their opponents -- the former will be prefixed by "home" and the latter will be prefixed by "opp."

Also included in this data set, but not used in the corresponding paper are longest kickoff return (kolong), number of points (points), rate of first downs (1rate), and turnover rate (torate = number of interceptions plus number of fumble recoveries, divided by time of possession).

``` Columns  Variable      Description
1 -   3 initials      team initials
5 -  26 team          name and location of the team
28 -  29 wins          wins
31 -  32 losses        losses
34 -  35 homedrives50  drives begun in opponents' territory
37 -  38 homedrives20  drives begun within 20 yards of the goal
40 -  41 oppdrives50   opponents drives begun in team's territory
43       oppdrives20   opponents drives begun within 20 yards of goal
45       homepuntblock punts blocked by team
47       opppuntblock  punts team had blocked
49 -  50 hometouch     touchdowns scored by team
52 -  53 opptouch      touchdowns scored against team
55 -  58 homeyards     total yardage gained by offense
60 -  63 oppyards      total yardage allowed by defense
65 -  68 hometop       time of possession by offense (in minutes)
70 -  73 opptop        time of possession by opponents' offense
75 -  76 homefgm       field goals made
78 -  79 oppfgm        field goals allowed to opponents
81 -  82 homefga       field goals attempted
84 -  85 oppfga        field goals attempted by opponents
87 -  89 opppuntno     punts made by opponents
91 -  94 opppuntave    average length of punts made by opponents
96 -  99 opppuntnet    average change in field position
during opponents' punts
101 - 102 opppunttb     opponents' punts taken for touchbacks
104 - 105 opppunt20     opponents' punts that resulted in the team's
offense beginning within 20 yards of their
own (defensive) goal line
107 - 108 opppuntlong   longest opponents' punt
110 - 112 homepuntno    punts made by team
114 - 117 homepuntave   average length of punts made by team
119 - 122 homepuntnet   average change in field position
during team's punts
124 - 125 homepunttb    team's punts taken for touchbacks
127 - 128 homepunt20    team's punts that resulted in the opponents'
offense beginning within 20 yards of their
own (defensive) goal line
130 - 131 homepuntlong  longest team punt
133 - 135 home1sts      first downs obtained by offense
137 - 139 opp1sts       first downs allowed by defense
141 - 142 homesacks     sacks achieved by team's defense
144 - 145 oppsacks      sacks allowed by team's offense
147 - 148 homekos       kickoffs made by team
150 - 151 oppkos        kickoffs received by team
153 - 156 homekoyds     yards gained during kickoff returns
158 - 161 oppkoyds      yards allowed to opposition during kickoff returns
163 - 166 homekoave     average yards gained during kickoff returns
168 - 171 oppkoave      average yards allowed during kickoff returns
173 - 175 homekolong    longest kickoff return made by team
177 - 179 oppkolong     longest kickoff return allowed by team
181       homekotds     kickoffs returned for a touchdown by team
183       oppkotds      kickoffs returned for touchdown by opposition
185 - 186 homerets      punts returned by team
188 - 189 opprets       punts returned by opposition
191 - 192 homefc        punts "fair caught" by team
194 - 195 oppfc         punts "fair caught" by opposition
197 - 199 homeretyds    return yardage on punts by team
201 - 203 oppretyds     return yardage on punts by opposition
205 - 208 homeretave    average length of punt returns by team
210 - 213 oppretave     average length of punt returns by opposition
215       homerettds    punts returned by team for a touchdown
217       opprettds     punts returned by opponents for a touchdown
219 - 220 homeint       interceptions made by team's defense
222 - 223 oppint        interceptions made against team's offense
225 - 226 homerecover   fumbles recovered by team's defense
228 - 229 opprecover    fumbles recovered by opposing defenses
231 - 232 numgames      games played by team
234 - 237 opprateyds    average number of yards gained
per minute of possession by opponents
239 - 242 homerateyds   average number of yards gained
per minute of possession by team
244 - 247 opppuntrate   average number of punts
per minute of possession by opponents
249 - 252 homepuntrate  average number of punts
per minute of possession by team
254 - 258 oppratetd     average number of touchdowns
per minute of possession by opponents
260 - 264 homeratetd    average number of touchdowns
per minute of possession by team
266 - 269 winpercent    winning percentage
271 - 275 hometorate    turnovers obtained by team,
per minute of possession by opponents
277 - 281 opptorate     turnovers allowed by team,
per minute of possession
283 - 286 home1rate     first downs obtained by team,
per minute of possession
288 - 291 opp1rate      first downs allowed by team's defense,
per minute of possession by opposition
293 - 295 homepoints    points scored by team
297 - 299 opppoints     points scored against team
301 - 303 conference    conference to which the team belongs (AFC or NFC)
```

Acknowledgements

The authors wish to acknowledge the assistance of our colleagues, Robert Shumway and Alan Fenech, for their helpful comments on a primitive version of this paper. We also thank the Department Editor, Roger Johnson, and two anonymous referees for their suggestions, particularly with respect to the graphs they recommended.

References

Anderson-Cook, C., Thornton, T., and Robles, R. (1997), "Measuring Hockey Powerplay and Penalty Killing Efficiency", in Proceedings of the American Statistical Association Section on Statistics in Sports, Alexandria, VA: American Statistical Association, 11-14.

Johnson, R. A., and Wichern, D. W. (1998), Applied Multivariate Statistical Analysis (4th ed.), Upper Saddle River, NJ: Prentice Hall.

Samaniego, F. J., and Watnik, M. R. (1997), "The Separation Principle in Linear Regression," Journal of Statistics Education [Online], 5(3). (ww2.amstat.org/publications/jse/v5n3/samaniego.html)

Venables, W. N., and Ripley, B. D. (1997), Modern Applied Statistics with S-PLUS (2nd ed.), New York: Springer Verlag.

Mitchell Watnik
Statistical Laboratory
University of California
Davis, CA 95616
watnik@wald.ucdavis.edu

Richard A. Levine
Department of Statistics
University of California
Davis, CA 95616
ralevine@ucdavis.edu