Friday, January 24, 2014

Friday Facts

John Gasaway has come out with this season's first full-fledged "Tuesday Truths"—his indispensable weekly breakdown of the conference-only efficiency margins for every team in every conference worth caring about.

But the Tuesday Truths can be deceptive. The efficiency margin is calculated like so: points scored minus points allowed, divided by number of possessions. There is no accounting for strength of opponent. By the end of the season, this matters little, as we can expect things to more or less even out. But early in the season, differing schedules can make a big difference.

So I took a stab at producing an adjusted efficiency margin for the Big Ten. For each game, I calculate the difference between a team's actual efficiency margin and the expected efficiency margin of an "average" Big Ten team against that particular opponent at that particular venue. That difference is adjusted efficiency margin. Add up each game for each team, and you get their total adjusted efficiency margin.

To illustrate, let's look at Iowa versus Wisconsin, at Wisconsin. The average Big Ten team—that is, a team with adjusted offensive and defensive efficiencies at the average of all Big Ten teams—would be expected to lose to Wisconsin at Wisconsin by 17 points per 100 possessions. In fact, Iowa lost by four points in a 70 possession game, which comes out to 6 points per 100 possessions. Thus, Iowa's adjusted efficiency margin for that game was +11 per 100 possessions, or 0.11 per possession.

On the flip side, the average Big Ten team playing Iowa at home would be expected to lose by 1.3 points per 100 possessions, and Wisconsin won by 6 per 100. So Wisconsin was above average too—+0.07 per possession.

When you add up each team's performances from all their games you get their total adjusted efficiency margins:

There's no real surprise at the top or very bottom of this chart. Michigan State, Michigan, and Iowa have clearly performed the best in conference play so far, and that shows up in the raw efficiency margins. But the adjusted margins give Iowa credit for its close losses at Wisconsin and at Michigan, and so Iowa just barely trails Michigan in the adjusted ranking.

Minnesota also gets credit for its tougher schedule and better wins. Although its raw efficiency margin is negative overall—placing it behind Purdue—the adjusted margin gets Minnesota ahead of Purdue and into the conference's upper division, where it clearly belongs based on its play so far.

The other notable outlier is Nebraska, which jumps ahead of three teams (Indiana, Illinois, and Penn State) at the bottom of the standings. Nebraska's raw efficiency margin gets killed by its big blowout loss to Ohio State, and doesn't reflect any credit for its respectably close losses to Michigan and at Iowa, or for the impressiveness of its win over Ohio State. These results are why Nebraska has actually moved ahead of Illinois in the Kenpom ratings, and the Adjusted EM accounts for them. 

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