But live sports, and particularly the N.F.L., have become increasingly defined by their television ratings. For the past decade, ratings across television have fallen because of expanding cable packages, competition from streaming services and DVRs, and the ubiquity of smartphones.

Live sports have proved mostly immune from this trend. Until now.

The N.F.L.’s regular season ratings fell by about 8 percent from 2015 to 2016, a drop that has been subject to all manner of explanation. Some believed news coverage of the polarizing presidential race siphoned off viewership, pointing out that ratings improved after the election. Others cited a dearth of stars on the field — a consequence of Peyton Manning’s retirement and Tom Brady’s suspension — and a lack of compelling games. Increased awareness of concussions and of the physical toll of football, high-profile cases of domestic abuse by players and disgust over protests during the national anthem all have been blamed, too.

The unsatisfying truth is that N.F.L. ratings are a Rorschach test, often revealing more about the test taker than the subject.

Ratings for the first week of this season dropped by 12 percent from last year, but those games were played the day Hurricane Irma landed in Florida, when millions of viewers were tuned to coverage of the storm instead of football. Ratings for Week 2 were down by 15 percent — but only by 4 percent if one factors in that the Thursday night game was only broadcast on the N.F.L. Network, while last year it was on both the N.F.L. Network and CBS, which has a wider reach.