Blog Entry

Week 7 probabilities

Posted on: October 21, 2010 8:35 am
 
One of the more intriguing matchups this weekend, and possibly the most puzzling prediction of the efficiency model all season, is the Patriots at Chargers. The Patriots come into Week 7 at 4-1, fresh off a dramatic win over the contending Ravens. The Chargers enter the weekend at 2-4, fresh off a loss to the rebuilding Rams. But somehow the model makes the Chargers heavy favorites over the Patriots  at 0.85 to 0.15.

In fact, the Chargers sit atop all other teams in the weekly rankings produced by the game probability model thanks to their passing efficiency on both sides of the ball. Philip Rivers and the rest of the offense are in a class by themselves, averaging 7.9 net yards per pass attempt. San Diego’s pass defense is also best in the league at 4.9 net yards per attempt. The Chargers’ running efficiencies on offense and defense are both better than average, as are their turnover rates.

So what is going on? How can a team that leads the league in efficiency (and in total yards) on both sides of the ball have only a 2-4 record to show for it? A big part of the answer is very clear: special-teams play. The Chargers have given up  three touchdowns on kicks and punts, and have had further difficulties on special teams.

But things still don’t add up, so let’s look at turnover differential. Although the Chargers are better than average in interception rate, they pass so often that they actually have a turnover differential of -3. This certainly isn’t good, but even combined with their special-teams failures, it still doesn’t fully explain four losses for such a statistically dominant team. Something else is going on.

I think a big part of the Chargers’ 2-4 record is bad luck. Statisticians might call it sample error or randomness, but whatever you call it, it’s not going well for San Diego. I’m not talking about leprechauns or superstitions or the random bouncing of footballs. (Although the Chargers have lost 9 of 11 fumbles, and the league-wide rate is about 50 percent. Fumble recovery is a notoriously random event in football — just look at the shape of the ball.)

Rather, I’m talking about a concept I call “bunching.”

Let’s say there are two baseball teams, completely equal in ability, playing one game at a neutral site. Each team performs perfectly equally, both hitting exactly nine singles over nine innings. But let’s say one team gets all its singles in one inning, and the other has its singles spread out one per inning. The first team might win, 7-0. It’s an extreme example, but it illustrates an overlooked point about many sports. Successful plays are not enough. Consecutive successes are required to win.

In football, two equal teams could each have 12 first downs in a game. One team could have three drives of four consecutive first downs, each leading to a touchdown, and the rest of its drives could be three-and-outs. The other team could have 12 drives consisting of one first down followed by a punt. Both teams could have equal yards, first downs and efficiency stats, and yet one team could win, 21-0. It’s easy to imagine a game in which one team has many more first downs and yards, but still loses. Could something like this bunching effect be cursing the Chargers?

It’s a given that N.F.L. offenses tend to score in proportion to their yards gained. It’s actually an extremely tight correlation, and the best–fit estimate of a team’s points per game is to take just under 10 percent of its yards per game and subtract 10. For the Chargers, who lead the N.F.L. with 433 yards gained per game, we’d expect the offense to score about 32 points per game, but they’ve actually scored only 26.

A similar analysis for the Chargers’ defense, with the special-teams scores set aside, shows that it has  allowed almost 2 points more per game more than the yardage total implies. That’s a total difference of 8 points per game.

If we could magically add those 8 points onto the scoreboard for each game this season, the Chargers would have five wins, no losses and a tie. Of course, things aren’t that simple, and we can’t just add points after the fact. But it’s an exercise that illustrates just how random game outcomes can be, even in the N.F.L.

Here are your Week 7 game probabilities:

Win ChanceGAMEWin Chance0.45Cincinnati at Atlanta0.550.37Washington at Chicago0.630.40St. Louis at Tampa Bay0.600.49San Francisco at Carolina0.510.19Buffalo at Baltimore0.810.45Philadelphia at Tennessee0.550.14Jacksonville at Kansas City0.860.52Pittsburgh at Miami0.480.28Cleveland at New Orleans0.720.13Arizona at Seattle0.870.15New England at San Diego0.850.19Oakland at Denver0.810.26Minnesota at Green Bay0.740.53Giants at Dallas0.47
Comments
The views expressed in this blog are solely those of the author and do not reflect the views of CBS Sports or CBSSports.com