Vegeta is testing whether running with weights improves people's speed relative to running without weights. He has one group of runners train by running three miles a day, five days a week wearing a 20 lb. vest, and has another group of runners train by running three miles a day, five days a week without a weighted vest. Then Vegeta has both groups run a three mile race to see how quickly they finish. In this example, Vegeta ______ race speed at the end of the study. measured manipulated mediated