Of the thousands of measurement scales that have been constructed, two critical questions are asked of each: “Is it reliable?“ and “Is it valid?“ The question of reliability (the topic of this chapter) addresses the issue of whether this instrument will produce the same results each time it is administered to the same person in the same setting. Instruments used in the sciences are generally considered reliable if they produce similar results regardless of who administers them and regardless of which forms are used. The tests given to Nehemiah were certainly reliable: The same tests given many times over several weeks would yield a 40-yard dash of about 4.2 seconds, a vertical leap of about 53 inches, and a bench press of about 355 pounds. But this raised the second question. The cognoscenti of football coined the phrase, “Yeah, but can he take a hit?“ You see, in track and field it is considered impolite to tackle your opponent while running a hurdles race! They acknowledged Nehemiah’s exceptional physical skills, but he hadn’t played football since high school. Were these measures of physical skill good predictors of his ability to play in the NFL? They were concerned about the validity of the measure used by the 49ers. In a general sense, validity asks the question, “Does it actually measure what it is trying to measure?“ In the case of Nehemiah, they wondered if measures of 40 yards speed, leaping ability, and strength actually measured his ability to play professional football.