A measurement is only as good as the reliability of the instrument, and the validity of its construction. Phrenologists thought mental faculties could be measured from skull shape. These are easy to measure reliably, but not sensibly related to what they were supposed to be measuring. Note - a test can be reliable, but not be particularly useful or sensitive to a phenomenon.
Most individuals score close to zero.
Most individuals score close to the maximum. Internal validity is the extent the study is addressing what it was intended to address. For example, if we were interested in the effects of age upon cognitive function, we might give people of different ages several computerised cognitive tasks. Confounding variables such as the age groups' differential exposure to computers may influence the study, as might differences in the amounts of education the groups have had. Confounding variables are variables that potentially impact upon results but are allowed to vary freely with the experimental variable of interest.