A tire company wants to determine if tires made with a new type of tread will last longer than tires made with the original type of tread. The company has access to 20 different vehicles and wants to use a randomized block design for the experiment. Which of the following variables would be the best to block the experimental units?

A blocking variable should group vehicles that are likely to have similar effects on tire wear. The type of vehicle—such as a truck, SUV, or sedan—can affect tire life because different types vary in weight and how they are driven. Blocking by type helps account for those differences when comparing the two tread designs.

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