Airlines may be motivated by financial concerns to underreport the percentage of flight delays caused by airline error.

The delays caused by uncontrollable factors could have led to an increase in complaints to airlines.

Complaints may not be the most reliable measure of how many errors occurred in a given year.

Delays caused by weather and other uncontrollable factors could have increased dramatically during the year under discussion.

Airline customers might not believe that particular delays were caused by uncontrollable factors rather than airline error