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The original was posted on /r/nfl by /u/geauxpackgo on 2024-09-01 21:19:30+00:00.
Inspired by a post by u/JPAnalyst earlier this week, I decided to do a deeper dive into the relationship between preseason and regular season success. I’ve heard it said that the best predictor of a teams success in any given season is simply the results from the previous season, and even more specifically point differential is a better predictor than win percentage. Taking those metrics as a point of comparison, I parsed through every teams preseason and regular season win percentage and point differential over the past 40 years to see what I could find.
TL;DR preseason success used to be fairly solidly associated with regular season success but that association now appears to be completely gone
The above plot shows how the positive association between preseason results and regular season success has decreased over time. In the mid-to-late 80s there was a solidly moderate correlation between preseason point differential and regular season win percentage. Over the subsequent 40 years the correlation has decreased to the point that in the past three years post-covid, there is absolutely no positive correlation between anything that happens in preseason and regular season win percentage. In fact, over those years preseason success is negatively associated with regular season success (although the magnitude of this correlation is so small as to be utterly meaningless). Interestingly enough, in the late 90s-early 2000s, preseason point differential was more highly correlated with win percentage than anything that happened the previous season.
In order to understand what’s going on, it may be helpful to give some context to interpret correlation or correlation coefficient. The correlation coefficient between two variables ranges from -1.0 to 1.0, and can be interpreted as follows:
We can visualize correlation via scatterplots of the two variables in question.
The above is an example of what the scatterplot of a moderate positive correlation looks like. It mostly just looks like a random smattering of points, but you can see that as the previous season’s point differential increases, the current seasons win percentage is a little likelier be higher as well.
Contrast that with preseason point differential, which is an example of a weak positive correlation. With the naked eye it just looks completely random, but believe it or not there is a slight positive relationship between preseason point differential and regular season win percentage over this 40 year span.