The 5 O'Clock Club: The difference between passer rating and quarterback rating

The 5 o’clock club aims to provide a forum for reader-driven discussion at a time of day when there isn’t much NFL news being published. Feel free to introduce topics that interest you in the comments below.

Two common measures that can be confusing

There are two statistics that are commonly used to measure quarterback performance:

• Quarterback Rating (QBR); and
• Passer Rating

Passer Rating is the official NFL statistic, while QBR was an ESPN invention designed to provide a more comprehensive evaluation of quarterback performance..

I have listed the top-15 quarterbacks of the 2017 regular season in the chart below. I have highlighted the top ranked quarterback for each rating system, as well as the Redskins 2017 starting quarterback, Kirk Cousins, and the Skins anticipated 2018 starting quarterback, Alex Smith.

Please note that the QBR numbers were taken from an ESPN website, while the passer rating stats were obtained from NFL.com, and a minimum of 100 pass attempts in the regular season was used as a cutoff for players to be included in the passer rating list.

Quarterbacks of interest

While Alex Smith is officially the highest ranked passer in the NFL for the 2017 regular season, ESPN gives the crown for overall quarterback play to Carson Wentz for the ‘17 regular season.

Smith is ranked 8th by ESPN, while the NFL’s passer rating has Carson Wentz at #5.

Kirk Cousins is ranked 14th in the NFL using the passer rating matrix, and 15th by ESPN’s QBR.

As has been mentioned by many people on Hogs Haven in the past few weeks, Alex Smith is coming off his best season as a passer in his 13-year career, while Kirk Cousins put up his worst season as a starter, both statistically, and in the win column.

Understanding the numbers

See the detailed explanations below for full understanding of the two quarterback performance matrices.

Quarterback rating (QBR)

Surprisingly, the best detailed explanation of quarterback rating and passer rating that I could find was from Wikipedia.

Raw QBR is calculated as the following:

where g() is a function that scales from 0-100, where 50 is average. Total QBR is the raw QBR adjusted for the strength of the opponent.

EPA is calculated based on the down, distance, and the yard line at snap, with each combination having its own point value. The point values are the average net point advantage the team on offense can expect given the particular down, distance, and field position. For example, a 1st and goal chance on the opponent’s’ 1 yard line heavily favors the offense, yielding a positive point value. On the other hand, a 3rd and 9 on the team’s own 3 yard line is heavily negative because it drastically favors the opponent.

The value of each play’s outcome is measured by the snap-to-snap change in expected points. This is called Expected Points Added. The Expected Points Added (or lost) in each play are divided among the contributing players on the field based on the role of each player and the type of play. Deeper throws give a higher share of credit to the QB, while screen passes give relatively less credit to the QB and more to the receiver.

Plays that occur in “trash time” are discounted by as much as 30%. Trash time is measured based on the leverage of each play which is primarily a function of score, time, and field position. Important, critical plays that are likely to change the outcome have high leverage, while plays that occur after the game has largely been already decided have low leverage. QBR discounts low leverage plays, but does not boost credit for “clutch” plays.

After each play’s Expected Points Added is adjusted for difficulty, division of credit, and trash time, it is averaged on a per play basis. This average is further adjusted to account for the strength of opponent. Performance against a stronger defense that tends to allow low adjusted EPA per play is adjusted upwards while performance against a weaker defense is adjusted downwards. The degree of adjustment is in direct proportion to the strength of the opponent.

Lastly, the resulting adjusted EPA per play is transformed to a 0 to 100 scale, where 50 is average. The result can be thought of as a percentile. For example, a QBR of 80 means that the QB’s performance is better than 80% of the game performances by QB’s since 2006. A game QBR of 80 would also mean that, given that QB’s performance, his team would be expected to win that game on average 80% of the time.^[5]

Think of QBR as an attempt to rate a quarterback by difficulty. A quarterback gets more credit for success in more difficult situations, and less credit for success in less difficult situations. It tries to reward QBs for making the clutch throws downfield versus an easy swing pass on second down.

It’s a 0 to 100 scale, where 50 should represent the true middle point, meaning those quarterbacks above 50 should be winning more games than those below 50. The nearer to 100 a QB is, the more ‘elite’ he should be.

Passer Rating

Passer Rating is the official NFL measure of quarterback performance, and its calculation is much simpler than the QBR, as it depends only on aggregate statistics rather than an analysis of each play a quarterback is involved in. Additionally, passer rating double counts completion percentage, favoring quarterbacks who tend to throw screens and other short passes. Passer rating is calculated using each quarterback’s passing attempts, completions, yards, touchdowns and interceptions, and has a maximum value of 158.3 and minimum value of 0.

Passer rating ignores large parts of a quarterback’s performance. It ignores sacks, fumbles, designed runs and scrambles. It also does not put plays into any context. For example, while the QBR treats a 5-yard gain on second-and-5 as very different from a 5-yard gain on third-and-10, passer rating treats all yards, whether they are air yards or yards after catch, as equal, and belonging to the QB.

The NFL passer rating formula includes four variables: completion percentage, yards per attempt, touchdowns per attempt, and interceptions per attempt. Each of those variables is scaled to a value between 0 and 2.375, with 1.0 being statistically average (based on league data between 1960–1970). When the formula was first created, a 66.7 rating indicated an average performance, and a 100+ rating indicated an excellent performance.^[3] However, passing performance has improved steadily since then and in 2017 the league average rating was 88.6.

The key here is that all plays are treated equally by the passer rating formula. There is no attempt to make one play more or less important than another. It also focuses on passing statistics, excluding plays a quarterback makes as a runner, or other non-passing measures.