Wednesday, October 30, 2013

Turnover Index - Week 9

Here are the Turnover Index picks for Week 9. The Turnover Index is a simple betting strategy based on the theory that the market overvalues defensive turnovers when judging team strength. See here and here for more background.

Week 8 results

Week 8 picks went 1-2 against the spread. Fortunately, the one successful bet was with the highest stake, so the overall loss to the bankroll was minimal. Here are the season to date results:
  • Against the Spread: 5-4
  • Starting Bankroll: $1,000
  • Current Bankroll: $867 (-13% ROI)
Here is the week by week performance:

weekbetswonstarting
bankroll
amount betprofitsending
bankroll
4 2 0 $1,000 $268 (26.8%) ($268) $731
6 1 1 $731 $32 (4.5%) $29 $761
7 3 3 $761 $130 (17.1%) $118 $879
8 3 1 $879 $70 (8.1%) ($12) $867

Sunday, October 27, 2013

NFL Playoff Seed Probabilities

Because it's never too soon to start talking playoffs.

The latest version of my NFL ranking table features some odd looking multi-colored bar graphs, taking the place of the monochrome Projected Wins bar graph. These new graphs represents each team's projected playoff seed probabilities, from #16 on the left to #1 on the right. The red bars are for seeds 16-7 (out of the playoffs) and the blue bars are for seeds 6-1 (wildcard and division winners). Mouseover the bar graph for the actual probabilities.

The probabilities are based on simulating the remainder of the season 10,000 times (see the Projected Wins post for more details). I then apply the NFL playoff seeding rules and various tiebreakers. As a reminder, the top 4 seeds belong to the four division winners, with the remaining two playoff seeds going to the two non-division winners with the best record. For a fully armed and operational NFL simulation tool, check out nfl-forecast.com.

This is virtually identical to a feature I added to my NBA rankings in February of this year. After the regular season was finished, I also set the probabilities in motion, something I plan on on doing for the NFL as well, once week 17 is complete. Some observations:

Wednesday, October 23, 2013

Turnover Index - Week 8

Here are the Turnover Index picks for Week 8. The Turnover Index is a simple betting strategy based on the theory that the market overvalues defensive turnovers when judging team strength. See here and here for more background.

Week 7 results

Week 7 picks went 3-0 against the spread. Here are the season to date results:
  • Against the Spread: 4-2
  • Starting Bankroll: $1,000
  • Current Bankroll: $879 (-12% ROI)
Here is the week by week performance:

weekbetswonstarting
bankroll
amount betprofitsending
bankroll
4 2 0 $1,000 $268 (26.8%) ($268) $731
6 1 1 $731 $32 (4.5%) $29 $761
7 3 3 $761 $130 (17.1%) $118 $879

Were it not for my pathological honesty, I would be reporting a 4-0 record against the spread this season, as the the initial two losses in week 4 occurred before I could start publishing the turnover index this season. As I was running the data for this season's version of the index, I debated whether to start betting in week 4 or week 5. But week 4 showed positive returns over the testing window (1999-2012), so I included it.

Despite the positive 4-2 record against the spread, my bankroll (managed according to the Kelly Criterion) is still underwater, having lost 121 of my starting 1000 (Zimbabwean) dollars.

Friday, October 18, 2013

Turnover Index - Week 7

Here are the Turnover Index picks for Week 7. The Turnover Index is a simple betting strategy based on the theory that the market overvalues defensive turnovers when judging team strength. See here and here for more background.

Week 6 results

Week 6 picks (or pick) went 1-0 against the spread (Chargers covered against the Colts). Here are the season to date results:
  • Against the Spread: 1-2
  • Starting Bankroll: $1,000
  • Current Bankroll: $761 (-24% ROI)
There is a table at the bottom of the post which summarizes performance by week in more detail.

Early Season Power Rankings - Follow Up

This is a follow up to last week's post on early season power rankings. In that post, I attempted to measure the accuracy of various NFL power rankings by comparing the team rankings after week 4 of the season to each team's remaining wins for weeks 5-16.

The original dataset was for seasons 2009-2012, and just looked at the week 4 rankings. I have now added seasons 2007 and 2008, plus a look at how the rankings after week 8 correlated with future wins. As a reminder, higher percentages are better, with 100% meaning perfect correlation between the ranking and future wins (the metric used is the Spearman rank correlation coefficient).

Week 5 Ranking Correlation to Future Wins
ranking average 2007 2008 2009 2010 2011 2012
espn 47% 55% 42% 51% 55% 43% 39%
dvoa 50% 57% 45% 47% 46% 41% 65%
ans 40% 50% 42% 51% 15% 49% 32%
market 54% 68% 36% 67% 45% 52% 56%
srs 52% 70% 46% 47% 38% 56% 54%

Week 8 Ranking Correlation to Future Wins
ranking average 2007 2008 2009 2010 2011 2012
espn 46% 58% 42% 51% 43% 41% 41%
dvoa 53% 77% 53% 59% 44% 35% 46%
ans 49% 70% 47% 57% 38% 39% 42%
market 55% 62% 50% 62% 56% 55% 42%
srs 53% 75% 51% 55% 46% 42% 50%


Keep it Simple

The performance of the Simple Ranking System (SRS) continues to impress, either matching or outperforming the more sophisticated DVOA and ANS models. The market ranking still appears to be the most accurate.

More data makes you dumber

Interesting to note that while all of the quantitative rankings improved going from week 4 to week 8 (particularly the Advanced NFL Stats efficiency model), the qualitative ESPN ranking took a small step backward. It's hard to read too much into such a small difference, but it is consistent with Ken Pomeroy's observations on the AP rankings for college basketball. For the 1990-2010 academic years, he compared tournament performance of the pre-season AP #1 to the AP #1 immediately prior to the tournament. In general the pre-season #1 fared better in the tournament (out of 21 seasons, the pre-season #1 won it all 6 times, compared to just 3 for the pre-tournament #1).

It's counter-intuitive, but has a simple explanation. As you get further into the season, most poll-based rankings tend to become retrodictive, rather than predictive. In other words, they "reward" teams for past wins, rather than judge them on their ability to create future wins. Bill Parcells statements not withstanding, a team's win-loss record is not always the best predictor of future success.

Saturday, October 12, 2013

The Turnover Index (possibly overfit edition)

Andrew Luck fumbles at 2009 Big GameThe Turnover Index makes a belated return this season, with some additional tweaks and enhancements. The Turnover Index is a simple betting strategy based on the theory that the betting market overvalues defensive turnovers when evaluating future team performance. I laid out the evidence in this post: NFL Turnover Differential and the Point Spread. Using data from the 1998-2011 seasons, I found that teams that had generated at least 10 fewer defensive turnovers (season-to-date) than their opponents covered the spread 58.7% of the time.

In a series of weekly posts, I tracked the performance of this betting strategy (bet on the team with at least 10 fewer defensive turnovers) throughout the 2012 NFL season. Initial results were promising, but regressed somewhat near the end of the season. At season's end, the strategy had gone 18-16-1 against the spread, for a whopping 1% return on in investment.

In this post, I will lay out a somewhat revised version of the turnover index which will allow a more sophisticated betting strategy based on the Kelly criterion for bankroll management.

Sunday, October 6, 2013

Early Season Power Rankings


When it comes to power rankings, week 5 makes for interesting times. We have just enough data (four games) to start questioning our pre-conceived notions of who the strongest and weakest teams are. Are the Chiefs as good as their undefeated record? Are the Steelers and Giants really as bad as their 0-4 marks? Once you get past the midpoint of the season, most power rankings, for better or worse, tend to coalesce around win-loss record. But with just four games, even the pundits try to be good little Bayesians when it comes to evaluating small sample sizes.

We tend to forget how off the mark our early season evaluations can be, so for this post, I decided to take a look at how various NFL power rankings evolved over past seasons, and, most importantly, how well those rankings correlated with future wins.