Thursday, January 5, 2012

Respect Randomness, K.I.S.S., Beat Vegas.

by Jim Glass

The 2011 end-of-season result is in for BigWin% - the team rating system that treats games decided by one score, 8 points or less, as ties in computing each team's W-L%, then adds a strength-of-schedule adjustment, that is all. (Rationale and background explained previously.)

Here are game prediction and efficiency results for the final eight weeks of the season. ("Efficiency" = correct game predictions divided by expected correct predictions. E.g., if among a week's 16 predictions the average predicted winner is a .750 favorite then 12 correct predictions would be expected. If the actual number is 10 then efficiency is .833, if 11 then efficiency is .917, etc.)

Big Win Index 82-40.6721.003
Las Vegas81-43.653.985

(Game counts don't match because of no-favorite predictions and "push" results.)

OK, this isn't really a method to "beat Vegas", that was just a bit of hyperbole.

But it's remarkable that such a simple, bare bones rating-and-picking system can so equally match Vegas while dismissing 51% of all game outcomes as being 50-50, split-the-difference, random chance. And while being applied purely mechanically, with no knowledge of teams being down to their third-string QBs, special match-ups, teams resting starters, or any of the other game-specific information known to Vegas.

Of the 126 games played during these eight weeks, 64 were decided by less than 9 points. That is 50.8%, or slightly more than the 42% of game outcomes that are determined by random chance according to Brian Burke. These game outcomes were dismissed, treated as "ties" by BigWin%.

Disregarding more than half of all game decisions may seem like the loss of a lot of information when evaluating teams, but sometimes less is more. Or, more may be less when what is considered more information really is just random noise.

The idea behind BigWin% simply was to filter out the 42% of game decisions that Brian says are decided by chance to get team W-L records much less distorted by chance. It works!

One can draw much larger lessons from this exercise about sport, life, fate, and the great amounts of work and resources that are spent in our time on parsing noise, but I shall resist.

For the record, here are the season-end BigWin% ratings of all the teams to make the playoffs:

RANKTeamBig Win%
1 GB0.776
2 NE0.743
3 NO0.710
4 BAL0.647
5 SF0.639
6 PIT0.623
7 HOU0.600
8 DET0.581
9 CIN0.580
... ......
12 ATL0.544
... ......
17 NYG0.507
... ......
25 DEN0.399

Yes, Denver is down at #25. BigWin% considers Denver's record during the 11 games with Tebow starting to be 4.5-6.5 (1-3 in two-score games plus seven close games divided 3.5-3.5) against .476 strength opposition, or .385 overall.

One of the life lessons exercises like this teach is to not confuse luck with a repeatable skill, but I've beaten that drum before.


Anonymous said...

Cool... I go solely by Y/PP with my rankings. And you know what? It has the same efficieny as Brian´s rankings or yours. Yes, yes, all that random turnovers (excluding the late "desperation mode" Int´s), starting QB´s who not really outperform the replacements (once they get enough practise reps), irrelevant rushing stats, etc., really can put some noise into rankings.

Karl, Germany

Pat Laffaye said...

I assume the chart is for the last 8 weeks. I computed the RAW BigWin% and see that the Saints were 5-0-2 and are at the top with 86%. After your SOS adjustment they fall to 3rd and 71%. That seems like a rather large drop. How did you adjust for SOS?

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