Every major sports outlet seems to offer N.F.L. power rankings. I admit my site offers them, too. The game probability model I use for these weekly predictions also ranks teams. I match each team up against a notional league-average opponent at a neutral site, and rank each team according to the resulting game probabilities.
There’s a lot of randomness in football game outcomes, and the model I favor is designed to ignore the noise and focus on the signal. This approach can sometimes produce curious results, as teams with relatively poor records are often ranked well ahead of teams with better records. You might attribute that to the inaccuracies and limitations of statistics used, and that’s partly true. But there’s much more to the discrepancy.
Consider a world in which all N.F.L. teams were perfectly evenly matched. Every game would be little different than the flip of a coin. In this kind of world, there’s still a very good chance there would be at least one 5-1 team and one 1-5 team, despite both being completely equal in team strength. In this hypothetical world, a power ranking based on team record would be an exercise in self-delusion.
The real N.F.L. isn’t as far off from the National Coin Flip League as you might think. Certainly there are better and worse teams, but there is also randomness in game outcomes. So although team records are not complete illusions, they are partially random and often misleading, especially early in the season.
Most power rankings of one kind or another are heavily influenced by team record, and it’s easy to understand why. Even the sharpest and most diligent sports pundits, who have the luxury of watching a wall of simultaneous games on Sunday, can only take in a portion of the action. And they remember even less — their brains aren’t Tivos. The previous weeks’ action is an even fainter memory. And what they do remember is probably biased by the information they have at their fingertips, which is game scores. So it’s not surprising that most power rankings closely match team records.
The illusion of power rankings goes deep. Even if power rankings are not necessarily good predictors of future performance, they can still be sound predictors of final team records and playoff qualification. The reason is the wins-in-hand effect. Season-ending records and playoff qualification is a function of two factors: 1) to-date wins in hand, and 2) future win expectancy. Power rankings that are overly influenced by to-date wins in hand will appear to be far more accurate by the end of the season than they really are. Even objective quantitative rankings that over-fit to the noise of past performance will intuitively seem to be a much better gauge of team strength that they truly are. If you’re grading someone’s rankings according to how well they match final records, you’re letting them cheat. They’ve glimpsed half the answer key from the teacher’s desk.
We already know factor #1. The team standings are there for anyone to see. The tough part is figuring out factor #2, and that’s what the game probabilities below are based on.
Here are your game probabilities for Week 7:
Brian Burke, a former Navy pilot who has taken up the less dangerous hobby of N.F.L. statistical analysis, operates Advanced NFL Stats, a blog about football, math and human behavior.
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