The recent decline in gas prices was generally expected to lead to a drastic decrease in demand for flights, based on the assumption that people have a limited amount of vacation time and would choose to spend it on road trips rather than airline travel. This assumption, however, is clearly false: A recent telephone survey revealed that a large majority of people who reported an increase in driving over the past year also reported no change in the number of flights they took during that same time period.
Which of the following would it be most useful to determine in order to evaluate the argument?