Understanding the Simple Rating System (SRS)
The Simple Rating System (SRS) is a team rating metric used on Pro Football Reference to measure how strong a team is relative to an average team in the league. Despite the "simple" in its name, SRS provides a surprisingly powerful snapshot of team quality by accounting for two key factors: a team's average point differential and the strength of the opponents they played.
At its core, SRS answers a straightforward question: how many points better or worse than average is this team? A rating of 0.0 represents a perfectly average team. A rating of +7.0 means the team is roughly seven points better than average on a neutral field, while a rating of -4.0 means the team is about four points worse than average. This makes SRS easy to interpret at a glance and allows for direct comparisons between any two teams; the difference between their SRS ratings is a rough estimate of the expected point spread if they played on a neutral field.
What sets SRS apart from simply looking at point differential is its adjustment for strength of schedule. A team that wins its games by an average of 10 points against weak opponents isn't necessarily better than a team winning by 5 points against tough opponents. SRS takes this into account by factoring in the ratings of every team a given team has played. Because each team's rating depends on its opponents' ratings, and those opponents' ratings depend on their opponents, the calculation works itself out through a system of interconnected equations that balance across the entire league.
Pro Football Reference uses a variation on the "traditional" SRS formula to account for teams that have played more/fewer games at home at various points in the season. The home-field advantage variable (HFA) changes from year to year based on game results, but it tends to make a 2 to 3 point difference in an average year. See
this blog post for more details on HFA.
SRS does have limitations worth noting. It treats all games equally regardless of when they occurred, so a team that improved dramatically over the course of a season will have a rating that reflects their full-season average rather than their current form, and because it's driven by point differential, blowout wins and losses can have an outsized influence. Even so, SRS remains a useful and intuitive tool for comparing teams across a season.