Regression Alert: Week 9 - Footballguys

Does Todd Gurley regress to the mean? I bet he does!

Welcome to Regression Alert, your weekly guide to using regression to predict the future with uncanny accuracy.

For those who are new to the feature, here's the deal: every week, I dive into the topic of regression to the mean. Sometimes I'll explain what it really is, why you hear so much about it, and how you can harness its power for yourself. Sometimes I'll give some practical examples of regression at work.

In weeks where I'm giving practical examples, I will select a metric to focus on. I'll rank all players in the league according to that metric, and separate the top players into Group A and the bottom players into Group B. I will verify that the players in Group A have outscored the players in Group B to that point in the season. And then I will predict that, by the magic of regression, Group B will outscore Group A going forward.

Crucially, I don't get to pick my samples, (other than choosing which metric to focus on). If the metric I'm focusing on is yards per target, and Antonio Brown is one of the high outliers in yards per target, then Antonio Brown goes into Group A and may the fantasy gods show mercy on my predictions.

Most importantly, because predictions mean nothing without accountability, I track the results of my predictions over the course of the season and highlight when they prove correct and also when they prove incorrect. Here's a list of all my predictions from last year and how they fared.


THE SCORECARD

In Week 2, I laid out our guiding principles for Regression Alert. No specific prediction was made.

In Week 3, I discussed why yards per carry is the least useful statistic and predicted that the rushers with the lowest yard-per-carry average to that point would outrush the rushers with the highest yard-per-carry average going forward.

In Week 4, I explained why touchdowns follow yards, (but yards don't follow back), and predicted that the players with the fewest touchdowns per yard gained would outscore the players with the most touchdowns per yard gained going forward.

In Week 5, I talked about how preseason expectations still held as much predictive power as performance through four weeks. No specific prediction was made.

In Week 6, I looked at how much yards per target is influenced by a receiver's role, how some receivers' per-target averages deviated from what we'd expect according to their role, and predicted that the receivers with the fewest yards per target would gain more receiving yards than the receivers with the most yards per target going forward.

In Week 7, I demonstrated how randomness could reign over smaller samples, but regression dominates over larger ones. No specific prediction was made.

In Week 8, I discussed how even something like average career length could be largely determined by regression-prone fluctuations in incoming talent. No specific prediction was made.

Statistic For Regression
Performance Before Prediction
Performance Since Prediction
Weeks Remaining
Yards per Carry
Group A had 24% more rushing yards per game
Group B has 4% more rushing yards per game
SUCCESS!
Yards:Touchdown Ratio
Group A had 28% more fantasy points per game
Group B has 23% more fantasy points per game
SUCCESS!
Yards per Target
Group A had 16% more receiving yards per game
Group A has 3% more receiving yards per game
1

Three weeks in and our yards per target prediction remains stubbornly intractable. Group A has gone from an 11.2 yard per game lead at the time of prediction to a 1.9 yard per game prediction since, but that last little gap is proving difficult to close. Group B will need a strong showing this week to avoid handing regression its first loss of the year.


Betting against Todd Gurley

Todd Gurley is having a monster season and everyone knows it. His 2018 so far marks just the fourth time in history that a player has scored 15 touchdowns through 8 games. When you consider that he also has three 2-point conversions, no player has ever matched his 18 trips to the end zone through eight games. He has outscored the No. #2 fantasy back, (James Conner), by 58.9 points in standard scoring, which is about how much James Conner has outscored RB15, (Isaiah Crowell).

We need a new prediction to track, and I really, really, really want to bet against him. If I was Captain Ahab and regression to the mean was my Pequod, then Todd Gurley would be my Moby Dick. (Yes, his obsession with the great white whale dragged Ahab to a watery grave, but it's not like allegory is an extended metaphor designed to teach us difficult truths about human existence or anything.)

The problem is one cannot merely bet against Todd Gurley. After all, he also leads the league in yards from scrimmage; even if we eliminated all touchdowns from every running back in the NFL, he'd still be the top fantasy back this year. Even a jaded regression-hound like myself must acknowledge that Gurley is clearly the top fantasy running back going forward, and it'd be folly to suggest otherwise.

But while I can't bet against him directly, there's nothing to stop me from rigging up a sample, brazenly placing him in Group A, and daring him to drag that cohort to victory. So let's do that.

There are currently fifteen running backs who have played at least four games and average at least 12 points per game in standard scoring. One of those backs, T.J. Yeldon, has been playing in place of an injured Leonard Fournette and is likely to lose his starting job in the coming weeks. I could try to shoehorn him into Group A to weigh down Mr. Gurley, but that hardly seems sporting, so let's set him aside. Another of those 15 is Todd Gurley himself. Let's set him aside for a moment, too.

This leaves us with 13 running backs averaging at least 12 points per game. Seven of these running backs— Marlon Mack, James Conner, David Johnson, Alvin Kamara, Kareem Hunt, James White, and Melvin Gordon III— have gained 110 yards or fewer for every touchdown they have scored. These high-touchdown backs will be our Group A. The other six running backs— Christian McCaffrey, Ezekiel Elliott, Tarik Cohen, Adrian Peterson, Saquon Barkley, and Joe Mixon— have gained 125 yards or more for every touchdown they have scored. These low-touchdown backs will be our Group B.

Collectively, Group A averages 91 yards for every touchdown while Group B averages 161 yards for every touchdown. As a result, the Group A backs have averaged 17.3 fantasy points per game vs. 14.8 points per game for Group B, an 18 percent advantage, (again, using standard scoring with no points per reception). Because touchdowns follow yards, I predict that Group B will outscore Group A going forward.

But wait, you say. What about Todd Gurley? Oh, very well. Mr. Gurley has averaged a touchdown for every 76 yards, the best rate in the league. If we include him in Group A, their average jumps all the way to 18.5 points per game, a 26 percent improvement over Group B. And yet even this mountain I believe is surmountable; I predict that Group B will outscore Group A going forward. Todd Gurley might be the best running back in fantasy, but I'll wager that even he isn't enough to single-handedly overcome the forces of regression and carry Group A across the finish line.

After all, if I've learned anything from literature, it's that nothing bad has ever happened to anyone who challenged the very forces of nature herself. Right?


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