An exercise physiologist classifies people— on the basis of their body mass index, heart rate, and lung capacity—as (a) above average in terms of fitness or (b) below average  in terms of fitness. He then directs the same people to walk on a treadmill, individually, at an increasing speed until they can no longer walk. The speed of the treadmill when a person maxes out on walking is the dependent variable. What statistical test should the physiologist use to see if there is a difference in maximum walking speed based on fitness level?