Power rankings are everywhere, but how do we know if they are any good? If they are intended to be predictive, we can test them for their predictive value. So, for the past three years, I have archived a cross section of power rankings from early in the NFL season (after week four). Then, once the season is done, I assess each ranking's accuracy in predicting future wins in that same season.
Each team's win percentage for weeks 5-16 (week 17 excluded) is rank ordered. This is then tested for agreement with each power ranking. I use the Spearman Rank Correlation coefficient for this purpose. A Spearman value of 100% would indicate perfect agreement between two rankings (e.g. if 1-32 in win percentage lined up identically with the power ranking in question). A value of -100% would indicate complete disagreement between the two lists, meaning 1-32 in win percentage lines up with 32-1 in the power ranking. A value of 0% would mean, roughly, no correlation between the two rankings.
I archived the week 4 power rankings in this post from October. As a reminder, the rankings I evaluate are: Football Outsiders DVOA, ESPN's FPI (as a replacement for Brian Burke's AFA efficiency rankings), the Simple Ranking System, ESPN's official NFL power rankings, FiveThiryEight's Elo rankings, and the Betting Market Rankings published here at inpredictable.
The table below ranks teams in order of week 5-16 win percentage, and lays that alongside where each ranking had that time after week four. The Arizona Cardinals and the Carolina Panthers led the league in win percentage over those 12 weeks, both going 10-1. All six rankings were fairly bullish on the Cardinals (average ranking of 4). The Panthers' strong season was more of a surprise, as Carolina had just an average ranking of 12 prior to their 10-1 tear.
The Tennessee Titans are at the bottom of the list, going 2-10. In general, the various ranking systems weren't too fond of the Titans, though the DVOA black box thought they were about an average team after week four.
|Weeks 5-16||Week 4 Rankings|
Scanning the table, each ranking system had its share of hits and misses. But the eye test can only get us so far, which is why we have the Spearman coefficient to make sense of it all.
The table below summarizes each ranking's Spearman coefficient for the 2015 season, adding it alongside the 2007-2014 results (reminder: a higher coefficient means a more accurate ranking):
|Week 4 Ranking Correlation to Future Wins|
Call shenanigans if you like, but for the third straight year, this site's betting market rankings performed the best when predicting future wins. And that same market-based ranking system has a commanding lead when averaged over the past nine seasons. Note that the 538 Elo rankings only have two season's worth of results, so the average is misleading.