Does listening to music while working out have an impact on the length of the workout? A sample of college students was randomly assigned to one of two groups. One group worked out on a treadmill...

Does listening to music while working out have an impact on the length of the workout?  A sample of college students was randomly assigned to one of two groups.  One group worked out on a treadmill while listening to rock music playing the background; the other group worked out on a treadmill with no music playing in the background.  It was found that those who worked out while rock music was playing spent more minutes on the treadmill, on average, than those who were working out with no music playing.    The explanatory variable in this experiment is