Why winning the NFL offseason can be a fallacy

The first wave of free agency is in the books and we’ve already seen a lot of splash moves, like the Indianapolis Colts trading for DeForest Buckner, the Arizona Cardinals trading for DeAndre Hopkins, the Miami Dolphins signing Byron Jones and the Cleveland Browns signing tight end Austin Hooper.

We’ve graded the Cardinals' and Dolphins' deals as “good” and the deals done by the Browns and Colts as “below average,” but all these deals have one thing in common: The respective teams expect their new players to have a huge impact in 2020, just as teams did this time last year.

However, reality often paints a different picture. Since 2006, players (excluding quarterbacks) who added value to their teams in a given season (i.e., they generated positive WAR) generated a total of 99 WAR less after they changed teams in free agency.

That’s roughly one-quarter of a win less per team and season than teams would have hoped for if they signed the new players hoping for just a replication of their previous year's success. 

First of all, that’s not really a surprising result because of the presence of regression to the mean, as a player who added positive value is generally expected to produce less value the following year. We also find this for players who don’t change teams, so a natural question to ask is whether or not the change of teams strengthens the regression.

As a first rudimentary analysis, let us bin players into three groups based on the previous year WAR and find the average WAR difference for players who switched teams and players who didn’t.

[Editor's Note: PFF's Wins Above Replacement (WAR) metric is powered by AWS machine learning capabilities.]

WAR difference for players with positive WAR in the previous season, excluding quarterbacks, 2006 – 2019