A group of sports-science researchers is trying to determine what physical characteristics are positive predictors for success at the quarterback position in high school sports. A thorough physical examination is planned for all high school participants, including ethnicity, height, weight. BMI, and other variables. One of the testing sites in the Midwest region places the measurement stick 4 inches off of the ground, generously adding to the height of each athlete in their training center. This mistake can be categorized as which kind of bias if not addressed by the researchers?