Stats in the NFL: Part 2 (in the wild)

In a previous post, I talked about parity in the NFL (or lack of parity) between teams. What about parity between divisions? What we want to do is test the null hypothesis that all divisions are the same (ie will end the season with the same number of wins.) To do this we can use the Pearson Chi-square test statistic.
In the table below, I have the year in the first column, the value of the test statistic in the second column, and the p-value in the third column. In all of the years, we never end up rejecting the null hypothesis of even strength between the divisions at the commonly used alpha=.05 level. However, we can use the p-values to look at relative parity among the divisions. A small p-value (in the case corresponds to a large test statistic) indicates that there is not parity among the divisions.

year v p
[1,] 2008 12.765625 0.07802902
[2,] 2007 8.937500 0.25717451
[3,] 2006 1.250000 0.98972973
[4,] 2005 2.437500 0.93172939
[5,] 2004 3.312500 0.85466795
[6,] 2003 1.062500 0.99375635
[7,] 2002 4.078125 0.77073626

Lets take a look at some of the interesting years.
In 2003, the win totals of the divisions was 36, 29, 34, 31, 31, 31, 31, 33. The first four are AFC East, North South, and West respectively. The last four are the NFC in the same order. The best division in the league only won 7 more games than the worst division.

Lets compare that with 2008. The wins totals by division were: 38, 31.5, 38, 23, 38.5, 25, 40, 22 (again in the same order as before.) The difference between the best division and worst division here is 12 games. And look at the miserable AFC and NFC west. 23 and 22 wins respectively. That should win those divisions a collective award for futility. Since 2002, (when the NFL moves to 8 divisions of 4 teams) the fewest wins for a division was 25. Two divisions managed to break that record in ONE season. And A third division tied that record.

Lets put into perspective how bad the AFC and NFC west were with year. The Detroit Lions went 0-16. Zero wins. And their division managed 25 wins this season. 2 more than the AFC west and three more than the atrocious NFC west. The west divisions are terrible.

That is why it makes me so angry that both the Chargers and the Cardinals won play-off games this past weekend. Will some one remind the chargers they started the season 4 – 8 and are champions of the woeful AFC west. And will someone remind the cardinals that they are the Cardinals.

Not convinced the NFC west sucks? How about this. The net points for the teams in the NFC west this season were:
Rams: -233
Seahawks: -98
49ers: -42

Let me repeat that. The Arizona Cardinals, winners of an actual NFL division and winners of a play-off game this year, outscored their opponents in the regular season 427 to 426.

Actually upon further inspection, in 2006, two teams made the play-offs with negative net points. The Giants were outscored by their opponents by 7 points and the Seahawks won the NFC west with a net total of -6 points on the season. In fact every team in the NFC west that year had negative net points. That is fairly impressive.

So congratulations to the Arizona Cardinals and the San Diego Chargers. Champions of futility. And also play-off winners.

Cheers.