A shoe company wants to determine if the new tread on its top line of running shoes lasts longer than the original tread. The company recruits 50 runners for a study. Each runner will perform their typical workout wearing one shoe with the original tread on one foot and another shoe with the new tread on the other foot. The foot that wears the new type of tread will be decided by flipping a coin. After one month, the runner will wear the new type of tread on the opposite foot. At the end of the second month, the difference in tread wear (New – Original) will be calculated.
This is paired data because each runner wears both tread types, one on each foot. The two measurements from a runner are linked, and the study compares the new and original tread within that same runner by calculating New − Original. The coin flip and switching feet help reduce the effect of foot differences.
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