In 2017, NFL teams completed 10,856 passes. They also threw 17,488 passes and took 1,195 sacks (totaling 18,683 dropbacks). That means NFL teams completed passes on 58% of all dropbacks — let’s call this adjusted completion percentage, since unlike regular completion percentage, it includes sacks in the denominator.

NFL teams also rushed 13,755 times relative to those 18,683 dropbacks. That means NFL teams passed (including sacks) on 58% of all plays. Let’s call this number the league pass ratio.

The fact that the adjusted completion percentage and pass ratio were the same in 2017 was not a coincidence. In 1981, the league-wide adjusted completion percentage was a lot lower at 51%.  That year, the league-wide pass ratio was also 51%.

In 1971, the adjusted completion percentage was 47 percent, while the pass ratio was 46 percent.

In 1962, the adjusted completion percentage was 49% and the pass ratio was 49%.

In other words, the adjusted completion percentage — which measures how often a pass play results in a completion — and the frequency with which teams pass are highly correlated and also nearly identical.  The graph below shows, in dark blue, the league average adjusted completion percentage, and in red, the league average pass ratio, for each year since 1950.

There are a few outliers.

• As I wrote about at length, the 1956 season was when the passing game was on life support.

• For a few years in the mid-1960s, the league pass ratio exceeded the adjusted completion percentage by about three points.  The league became a bit more pass-happy at this time, perhaps a reflection of competition with the AFL (this data is NFL-only, but the AFL’s pass-happy nature had an impact on the NFL, too) along with the presence of a number of great quarterbacks being active at the same time.

• Then, in the mid-’70s, the pass ratio dropped quite a bit and went south of the pass ratio line.  This was because pass efficiency sank during this time, even if pass completion percentage didn’t quite decline to the same degree.  Passer rating dropped from the high 60s to the low 60s in the middle of this decade.

• From 1985 to 1999, the average adjusted completion percentage was 52.5%, while the pass ratio was 54.8%.  Teams were passing more, but they hadn’t quite yet adopted the high-percentage passing game in modern times, but now, adjusted completion percentage and pass ratio have stabilized.

What stands out to you?