The NFL rates its passers for statistical purposes against a fixed performance standard based on statistical achievements of all qualified pro passers since 1960. The current passer rating system, which was adopted in 1973, removes inequities that existed in the former method and, at the same time, provides a means of comparing passing performances from one season to the next.
It is important to remember that the system is used to rate passers, not quarterbacks. Statistics do not reflect leadership, play-calling, and other intangible factors that go into making a successful professional quarterback.
Four categories are used as a basis for compiling a rating:
Percentage of completions per attempt
Average yards gained per attempt
Percentage of touchdown passes per attempt
Percentage of interceptions per attempt
The average standard is 1.000. The bottom is .000. To earn a 2.000 rating, a passer must perform at exceptional levels, i.e., 70 percent in completions, 10 percent in touchdowns, 1.5 percent in interceptions, and 11 yards average gain per pass attempt. In order to make the rating more understandable, the point rating is then converted into a scale of 100, with 158.3 being the highest rating a passer can achieve. In cases where statistical performance has been superior, it is possible for a passer to surpass a 100 rating.
Quarterbacks are required to throw at least 1,500 passes before their ratings qualify for NFL career statistics for the regular season, and 150 attempts for the postseason.