Item CV022002: In an experiment involving skateboards, in which the size of the wheels and the material the skateboards are made of are both varied at the same time, it is not possible to find out if either of these variables affects the performance of the skateboard.
A student wants to buy a new skateboard. He wants to find out if the size of the wheels affects how far he can coast on the skateboard. He also wants to find out if the type of material the board is made of affects how far he can coast on the skateboard. He decides to compare two skateboard models that are the same size but are made of different materials and have different size wheels:
He pushes off as hard as he can and stands on the skateboard until the skateboard comes to a stop. He tries each skateboard 5 times to see how far he can go. He uses the same pavement and the same starting point for all the trials. He finds out that he can coast farther with Model 1. What can he conclude from this test?

Distribution of Responses Percent of students responding correctly
 