This weekend, the NFL preseason kicks off in Canton with the Colts and Packers. Also, the Olympics, International Champions Cup, and the Indiana State Fair are happening. So, it’s a busy time in sports and artery destruction. So definitely it’s going to be entertaining, and maybe it will impact your health in 20 years (I can dig some football, futbol, sports only on TV every four years, and random fried things). But, as you see your favorite NFL team founder, excel, or be somewhere in the middle over the next month, how much should you draw from that in terms of the next few months of your life?
Some may say very little because it’s 4 (or 5, for the Hall of Fame Game teams) games, mostly featuring back ups. And of course, the 2008 Detroit Lions managed to win every preseason game and lose every regular season game. But that is hardly a deep empirical analysis. Of course, with the limited sample size, a strict, actual winning percentage is a bad measure of team performance. Winning half your preseason games and winning 3 out of 4 is a rather similar performance, much more similar than regular season records with the same winning percentages, 8-8 and 12-4.
So, how can the performances of a team in the preseason and regular season? One tool, originating in baseball and adapted to football by Football Outsiders, is the Pythagorean expectation, which uses the points scored by and against a team to predict the record (in this case, I’m using winning percentage because they can be comparable independent of number of games played) of a team. Since there is a greater variation in those values between teams in preseason, it should allow a more realistic range of potential results. And, because the values are also very easily able to be calculated for both the preseason and regular season, it should allow a large number of data points to be compared. For football, the equation is as follows:
Pythagorean Expectation=((Points for)^2.37)/((Points for)^2.37+(Points against)^2.37)
So, how does the preseason stack up as a predictor? Using the NFL’s data from the 2006-2015 seasons, this comparison can be easily performed (well, aside from a lot of copying and pasting from NFL.com). Not particularly well, actually. Well there is a slight correlation (a Pearson coefficient of .261), but as can be seen below, it is much more of noise than a pattern.
There are a mix of teams that excelled in the preseason and tanked in the regular season (2008 Lions), did poorly in the preseason and well in the regular season (2011 San Francisco 49er’s), and did roughly the same in both (2013 Seattle Seahawks).
So if your favorite team is doing very poorly or very well, it means nothing, right? Not exactly. There is a significant (Pearson of -.684) between a team’s preseason Pythagorean expectation and the difference between the preseason and regular season record. As shown in the graph below, this effectively means that teams that do either very well or very poorly in the preseason are likely to not be as big of a success or failure in the regular season.
Teams, on average, trend to being pretty average. So if your favorite team absolutely faceplants during the preseason, don’t be alarmed. They will probably not be terrible, just most likely mediocre. And potentially a playoff team. Which is better than the alternative. Sorry Lions’ fans.
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