Problem
Mike and Idan are interested in how cognitive functioning affects physiological fatigue. Mike collects data from 10 athletes boxing in Winter Springs's UFC gym. Idan collects data from 10 athletes who agree to practice in UCF's Institute of Exercise Physiology and Wellness lab. Unfortunately, they find that the average heart rates for those tested in the laboratory are different from those tested in the gym. Given that the physiological measures are different depending on the location of data collection, they fear that the correlations between cognitive and physiological measures will be inconsistent too. What threat to validity should Mike and Idan be most concerned with?