Dallas Cowboys Stat School: Passer Rating 101

Most Cowboys fans know that Tony Romo regularly has one of the highest passer ratings in the league.

There are Cowboys fans out there who know that Tony Romo holds the third best career passer rating of all time in the NFL. Here are the top five in career passer rating: Steve Young: 96.8, Philip Rivers: 95.8, Tony Romo: 95.6, Peyton Manning: 95.2, Kurt Warner: 93.7. Mighty fine company.

I would bet that the majority of BTB-members know how the passer rating is calculated (or at least know where to look it up), and we’ve all sort of developed a feeling that a number approaching 100 is pretty good, 80ish is about average and anything close to or below 60 is approaching JaMarcus Russell territory.

But most of us would probably be hard-pressed to explain what the passer rating means. Follow me after the jump, and next time somebody asks you what the passer rating means you won’t have to bow your head in shame or resort to some complex algebra to explain it. Read to the end and become the king of the water cooler by being able to explain the passer rating without a single formula. And don’t worry, there is no test at the end, just a simple multiple choice poll.

The passer rating was initially presented by Don Smith in 1973, then working for the Pro Football Hall of Fame, and has been used ever since. The passer rating combines four different components or efficiency measures into one number: completion percentage (CMP%), average yards per attempt (YPA), touchdown percentage (TD%), and interception percentage (INT%).

The intention of the passer rating was to give equal weight to all four components and to have a sliding scale that would differentiate between outstanding, poor and average performance. A league average passer performance would result in a score of 1.0 for each component. An outstanding (or record-breaking) performance would score 2.0 points, a really poor showing would net 0 points.

For the completion rate, this looked as follows: based on league averages in the early 1970s, the ‘average passer performance’ was set at a completion rate of 50%. These 50% would therefore equal 1.0 points in the formula. The highest completion rates up to that point were around 70%, so any passer completing 70% of his passes would get 2.0 points. A 30% completion rate would result in 0 points.

So how do you turn 70%, 50% and 30% into 2, 1 and 0 points respectively? You subtract 30 and then divide by 20. That is how the completion rate is calculated in the passer rating. A similar conversion formula was used for the other three components to make them fit the 2, 1 and 0 point logic. Below are the values that denote outstanding, average or poor performance in the passer rating, as implemented based on league averages in the early 1970s.

An average performance across all four components would therefore add up to 4 total points. A performance that would have been considered excellent in the early 1970s would result in about 6 points. It was felt at the time that making an excellent performance result in a passer rating of 100 would make the formula more user friendly. So after calculating all the individual components, the passer rating multiplies the sum by 100 and divides the total by 6. The result: an average performance (4 points) gets a 66.7 passer rating, an excellent performance by early 1970s standards (6 points) gets a 100 passer rating, an outstanding (or record-breaking) performance (8 points) gets a 133.3 passer rating.

Evolution of the passer rating

Using the league averages of the early 1970s, the passer rating formula was constructed in such a way that each of the four components were balanced and each contributed about 25% to the total passer rating.

For reasons outlined in the previous post (Passer's Paradise), the passing game in the NFL has undergone significant changes, and the balance in the passer rating formula has also changed significantly over the years. The graph on the right shows how the individual components of the passer rating have diverged over the years.

In 2009, completion percentage (31%) and interception rate (32%) accounted for almost two thirds of the passer rating, while yards per attempt (20%) and touchdown rate (17%) have lost significant weight in the formula. The passer rating today rewards the low risk, high completion game typical of a West Coast style offense and also reflects the increased involvement of running backs and tight ends in the passing game.

The initial goal of the passer rating was to establish a standard by which you can compare quarterbacks, regardless of when they played. And that goal has been achieved. All quarterbacks in the history of the NFL are measured by the same criteria, even though those criteria are based on 1970 standards.

It is actually fairly easy to adjust the passer rating formula to where each component is again weighted at about 25% to better reflect the average performances in the modern passing game. But that would defeat the purpose of having a comparable number across the decades.

Now that we’ve established the passer rating, warts and all, our next session "Peerless Passers" will be about the Cowboys’ quarterbacks and how they compare statistically. Thank you for your attention, class, and I hope this session proved to be enlightening.

Now if I could only find my glasses, the short bus has already pulled up outside …

Hat tip to kentuckybronco from SB Nation's MileHighReport.com for the poll questions template.