All football historians know about the 1978 rules changes. Here’s an except I wrote about how those changes changed the NFL forever:

There were two key rules changes enacted in 1978. The first prohibited bumping, chucking, or otherwise making anything other than incidental contact with a receiver beyond five yards from the line of scrimmage. The second allowed offensive lineman to be able to extend their arms, and push with open hands, allowing for better blocking and fewer holding penalties. With those rules in place, quarterbacks needed fewer blockers and receivers needed to be less skilled to get open. As a result, three and four wide receiver sets become more common, and the fullback was phased out…. From 1970 to 1977, non-starting wide receivers consistently produced just under 10% of the team’s total receiving yards; by 1990, that number had doubled, and has shown no signs of subsiding.

So what was the impetus for those changes? The 1977 season, which was a passing nightmare. In 1976, teams averaged 4.07 Adjusted Net Yards per Attempt, defined as (Gross Passing Yards + 20 * TDs – 45 * INTs) divided by (Pass Attempts – Sack Yards Lost). Now a 4.07 league average ANY/A wasn’t high – the ’60s NFL saw an average ANY/A of 4.59 — but it wasn’t notably low, either. In fact, 1976 was a slightly better passing environment than the trailing five year average of 4.01. Then, 1977 happened: teams averaged just 3.55 ANY/A, the lowest mark in the NFL since 1953. This was a dramatic decline in passing production of 0.52 Adjusted Net Yards per Attempt.

Fast forward 40 years. In 2016, the league-wide ANY/A average was 6.22, a shade off of the 6.26 ANY/A average in 2015. In fact, 2014 (6.14 ANY/A), 2015 (6.26), and 2016 (6.22) are the three best passing seasons in NFL history. There was little reason to expect 2017 to be any different, but it was: the NFL average ANY/A dropped to 5.91 last season, a decline of 0.31 ANY/A. That was the single largest year-over-year decline since 1976-1977.

But what’s most interesting to me is why ANY/A went down this year. There are a number of component parts to ANY/A. Completion percentage and yards per completion, when combined, make up yards per attempt. Sack rate helps convert yards per attempt to net yards per attempt. And TD rate and INT rate convert NY/A to ANY/A. So with those five variables, the big question should be which one caused the decline? The boring answer: all of them. ANY/A dropped by exactly five percent, from 6.22 to 5.91. What about the component parts?

Completion percentage dropped by 1.47%, going from 63.00% to 62.08%.

Yards per completion dropped by 0.47%, going from 11.35 to 11.30.

Sack rate increased by 11.1%, jumping from 5.8% to 6.4%. ^[1]Yards per sack was pretty consistent, going from 6.46 to 6.54.

TD rate decreased by 1.37%, falling from 4.30% to 4.24%.

INT rate increased by 8.4%, going from 2.27% to 2.46%.

But let’s look at it another way: what would the 2017 ANY/A numbers have been if we moved one of the five variables back to the 2016 levels but kept the other four at their 2017 levels?

If completion percentage stayed at 63.00%, but all other variables stayed the same, 2017 ANY/A would have been 6.00.

If yards per completion stayed at 11.35, but all other variables stayed the same, 2017 ANY/A would have been 5.94.

If sack rate stayed at 5.8% ^[2]And yards per sack stayed at 6.46., but all other variables stayed the same, 2017 ANY/A would have been 5.99.

If TD rate had stayed at 4.30%, but all other variables stayed the same, 2017 ANY/A would have been 5.92.

If INT rate had stayed at 2.27%, but all other variables stayed the same, 2017 ANY/A would have been 5.99.

So if you wanted to isolate the biggest reasons for the decline in ANY/A, it would be a three-way tie between lower completion percentage, higher sack rate, and a higher INT rate. What’s interesting is that those variables don’t usually move in the same direction — quarterbacks can trade risky throws for more sacks (lowering INT rates but increasing sack rates), or a lower completion percentage for fewer sacks (by throwing passes away instead of waiting for a play to develop, a QB can have a lower completion percentage but also a lower sack rate), so the decline is certainly reflecting a material decline in not just style, but quality of play.

Who knows what this will mean for 2018, but it will be very interesting to see the league-wide response. My guess is 2018 ANY/A will be 5.98. What do you think?