An automotive engineer wants to see if the brand of tire makes a difference in stopping distance.

She has new sets of Brand A tires placed on each of four cars of different makes and measures the stopping distances after the cars had reached a speed of 55 miles/h. Then she repeats the experiment with Brand B, then Brand C, and then Brand D. She concluded that the brand with the smallest mean stopping distance was the best tire. Discuss this from an experimental design perspective.