Francine rolled a marble down a ramp and timed how long it took for the marble to reach the bottom of the ramp. After one trial, she doubled the height of the ramp and added a thick layer of wool cloth to the surface of the ramp. She conducted three more trials and after reviewing the data, Francine concluded that the height of the ramp had no effect on the speed of the marble rolling down the slope. Which of the following reasons best describes why her conclusion is unsupported?