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 touchdown rate, and Christian McCaffrey is one of the high outliers in touchdown rate, then Christian McCaffrey 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 my predictions from 2019 and their final results, here's the list from 2018, and here's the list from 2017.
In Week 2, I opened with a primer on what regression to the mean was, how it worked, and how we would use it to our advantage. No specific prediction was made.
In Week 3, I dove into the reasons why yards per carry is almost entirely noise, shared some research to that effect, and predicted that the sample of backs with lots of carries but a poor per-carry average would outrush the sample with fewer carries but more yards per carry.
In Week 4, I talked about how the ability to convert yards into touchdowns was most certainly a skill, but it was a skill that operated within a fairly narrow and clearly-defined range, and any values outside of that range were probably just random noise and therefore due to regress. I predicted that high-yardage, low-touchdown receivers would outscore low-yardage, high-touchdown receivers going forward.
In Week 5, I talked about how historical patterns suggested we had just reached the informational tipping point, the time when performance to this point in the season carried as much predictive power as ADP. In general, I predicted that players whose early performance differed substantially from their ADP would tend to move toward a point between their early performance and their draft position, but no specific prediction was made.
In Week 6, I talked about simple ways to tell whether a statistic was especially likely to regress or not. No specific prediction was made.
In Week 7, I speculated that kickers were people, too, and lamented the fact that I'd never discussed them in this column before. To remedy that, I identified teams that were scoring "too many" field goals relative to touchdowns and "too many" touchdowns relative to field goals and predicted that scoring mix would regress and kickers from the latter teams would outperform kickers from the former going forward.
In Week 8, I noted that more-granular measures of performance tended to be more stable than less-granular measures and predicted that teams with a great point differential would win more games going forward than teams with an identical record, but substantially worse point differential.
In Week 9, I talked about the interesting role regression to the mean plays in dynasty, where the mere fact that a player is likely to regress sends signals that that player is probably quite good and worth rostering long-term, anyway. No specific prediction was made.
In Week 10, I explained why Group B's lead in these predictions tended to get smaller the longer each prediction ran and showed how a small edge over a huge sample could easily be more impressive than a huge edge over a small sample. No specific prediction was made.
In Week 11, I wrote that yards per pass attempt was an example of a statistic that was significantly less prone to regression, and for the first time I bet against it regressing.
|Statistic for regression||Performance before prediction||Performance since prediction||Weeks remaining|
|Yards per Carry||Group A had 3% more rushing yards per game||Group B has 36% more rushing yards per game||Success!|
|Yard to Touchdown Ratio||Group A averaged 2% more fantasy points per game||Group B averages 40% more fantasy points per game||Success!|
|TD to FG ratio||Group A averaged 20% more points per game||Group B averages 36% more points per game||Success!|
|Wins vs. Points||Both groups had an identical win%||Group B has a 4% higher win%||Failure|
|Yards per Attempt||Group B had 14% more yards per game||Group B has 34% more yards per game||3|
Group B entered the final week of our team wins prediction in good shape, but things went off the rails at the end. Other than four intra-group games (where it tautologically went 2-2), Group B went 1-3 in its remaining games, while Group A went 4-1. Worst of all, Group B was 0-3 in games against teams from Group A.
Had it won a single one of those games, it would have hit the 10% threshold needed to succeed. Was this a bad beat, then? No; had Group A won one more cross-group matchup, it would have finished with a better winning percentage than Group B entirely. Luck runs both ways.
Did the bias I identified last week (where games between two teams in the same group pulled each group's winning percentage closer together) make a difference? Yeah, but not enough to matter; removing games within the group only boosted Group B's edge to 5%, still well short of the 10% edge I needed to declare victory. Nope, this was just a good old-fashioned loss for the forces of regression. You can't win them all.
Our other prediction had a strong opening week to help ease the sting. At the time of the prediction, Group A was throwing significantly more passes, but Group B was averaging more yards per game thanks to a much higher yard per attempt average. Last week... Group A threw significantly more passes, but Group B averaged more yards per game thanks to a much higher yard per attempt average. Going forward, I'd expect more of the same.
Setting The Pace
Let's talk about "on pace" stats. For example: through eight games, Russell Wilson had thrown 28 touchdown passes, which left him on pace to throw 56 over a full season, which would have been a new NFL record.
I love pace stats. They're fun. I love football because it tells a story, and I think record chases are a big part of that narrative fabric. I love the "will he or won't he" aspect, the suspense, the anticipation.
I love looking at young players and imagining that pace stats give us a sneak peek at what we'll be saying about them this offseason. Justin Jefferson is on pace for 1350 receiving yards. At 18.8 yards per reception! What will we say about him this offseason if he finishes with those kinds of numbers?
But as much as I love them because of the stories they tell, I also recognize that they're just that: stories we tell. Regular readers are probably unsurprised that Russell Wilson has fallen off of his record-setting pace. He's thrown just two touchdowns in the last two weeks and is now "only" on pace for 48 on the season— a wildly impressive total, but not a record.
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