Two years ago, I wrote a six-part series on adjusting passer ratings for era.

• Part I
• Part II
• Part III
• Part IV
• Part V (Career Passer Ratings)
• Part VI (Single Season Ratings)

Last year, after the 2017 regular season ended, I posted the single-season results and also updated the career ratings. Today is the 2018 update.

Passer rating is made up of four variables: completion percentage, yards per attempt, touchdown percentage, and interception percentage. The reason passer rating needs to be adjusted for era? Well, that’s pretty simple to explain.

When the formula was derived in the early ’70s, an average rating in each variable was achieved with a 50% completion rate, averaging 7.0 yards per pass attempt, a 5% touchdown rate, and a 5.5% interception rate.  Since those numbers are wildly out of date, I came up with a formula that perfectly matches the intent of passer rating but ties the variables to the league average in any given season. You can get the formulas and read more background in the linked posts.

In 2018, the four averages were 64.9%, 7.37, 4.79%, and 2.37%. The big changes, of course, are in completion percentage and interception rate; yards per attempt is much more stable throughout history (although 2018 was higher than in recent years), while touchdown rate is actually slightly lower than it was in the ’70s.

One thing to keep in mind: these adjustments will not change the order of passer ratings in a given season. So Drew Brees, Patrick Mahomes, Russell Wilson, Matt Ryan, Philip Rivers will remain your top 5 leaders; the way the formula works, it simply subtracts a fixed amount from each passer’s actual passer rating. In 2018, that amount was an enormous 26.3 points from each passer.

Below are the 2018 passer ratings:

Josh Rosen had an actual passer rating of 66.7, which is what the average passer rating was designed to score in 1973. Rosen completed 55.2% of his passes, averaged 5.8 yards per attempt, threw a touchdown on 2.8% of his passes and an interception rate of 3.6%. Remember, the way the formula works is you calculate a grade for all four variables, and 1.00 is average; you sum the result, divide it by 6 and then multiply it by 100, which is why an average grade would be 66.7.

Without adjusting for era, Rosen’s 55.2% completion rate is worth 1.26 points, since it’s a bit better than 50%. His yards/attempt average is not very good, and translates to 0.70 points (the fact that his awful yards/attempt average is nearly offset by his completion percentage is a separate issue with passer rating, which is that is wildly overvalues completion percentage). His TD rate was terrible, and gets only 0.56 points, but his INT rate was great by 1970s standards, and translates to 1.48 points. So Rosen’s four variables sum to 4.0 by traditional passer rating standards, which is why he had a 66.7 passer rating in 2018.

Now, what about the era-adjusted numbers? Rosen’s completion percentage variable drops to 0.52, as he was nearly 10% below average. His yards/attempt average also drops, now to 0.61 points, when compared to a 7.4 average. His TD rate variable actually jumps up slightly, since the league average in ’18 was below 5.0%, to 0.60. But the INT rate variable is where he really drops: whereas a 3.6% rate was worth 1.48 points when compared to a 1970s basline, it’s worth 0.70 points compared to the 2.4% baseline of 2018. So the four variables now drop to being worth 2.43 points in the aggregate, which translates to a 40.45 era-adjusted passer rating, barely ahead of the disastrous 40.2 EA-PR DeShone Kizer posted in 2017.

The worst three passer ratings of 2018 all belong to rookies, but Rosen’s was the clear worst of the bunch.