Item CV001003: A carts and ramp experiment in which the weight of the carts is varied and the height of the ramp is held constant could tell you the effect of the weight of the cart on the cart's speed.
A student wants to know if the weight of a cart affects its speed at the bottom of a ramp. He can change the weight of the cart by adding different numbers of balls, and he can change the height of the ramp by using different numbers of blocks. Which set of tests should he compare (set A, B, C, or D)? 
Distribution of Responses Percent of students responding correctly
 
Modifed from TIMSS item population 2 (I12), 1994
