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Regression Alert: Week 5 - Footballguys

Revisiting preseason expectations and seeing what we've learned.

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.

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 8% more rushing yards per game
2
Yards:Touchdown Ratio
Group A had 28% more fantasy points per game
Group B has 69% more fantasy points per game
3

"Yards per carry is completely random", exhibit #103,962,411: in week 4, our "high-YPC" cohort averaged 4.219 yards per carry. Our "low-YPC" cohort averaged 4.224 yards per carry. Yards per carry is totally not actually a thing.

Also, if you were hoping for a dramatic result, our yard-to-touchdown ratio prediction last week certainly delivered one. Consider: over the first three weeks, our high-touchdown cohort scored 36 touchdowns in 39 player-games, or 0.92 touchdowns per game. Our low-touchdown cohort scored 4 touchdowns in 36 games, or 0.11 touchdowns per game.

In Week 4, 66% of "low-touchdown" players reached the end zone, and the group collectively scored 0.75 touchdowns per player game. Meanwhile, only 23% of "high-touchdown" players reached the end zone, and the group collectively scored 0.38 touchdowns per game, about half as many as Group B scored.

There's still a long way to go on both predictions, but the early returns show why I chose to focus attention here first.

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