A government agency publishes ratings of airlines, ranking highest the airlines that have the smallest proportion of late flights. The agency' s purpose is to establish an objective measure of the relative efficiency of different airlines' personnel in meeting published flight schedules. Which one of the following, if true, would tend to invalidate use of the ratings for the agency's purpose? ['Flights are defined as "late" only if they arrive more than fifteen minutes past their scheduled arrival time, and a record is made of how much later than fifteen minutes they are.', 'Flights are often made late by bad weather conditions that affect some airlines more than others.', 'Airline personnel are aware that the government agency is monitoring all airline flights for lateness.', 'Travelers sometimes have no choice of airlines for a given trip at a given time.']