Item CV010003: In an experiment involving balls and ramps, if both the height of the ramp and the size of the ball are changed at the same time, no conclusion can be reached about the effect of either of the individual variables on how far the ball can roll.
Students have a small steel ball and a large steel ball, and they have a short ramp and a tall ramp.
The students want to find out if a ball's size affects how far it can roll. They also want to find out if the height of the ramp affects how far the ball can roll. They do the following experiment:
What can they conclude from this experiment?

Distribution of Responses Percent of students responding correctly
 