When scores are measured on an interval or ratio scale of measurement, the mean is the most common measure of a distribution's "center" because its balancing of deviation scores provides a more precise measurement than the mode or median. In addition to describing a distribution's center, you also need a precise measure of a distribution's variability, or how much scores tend to deviate from the center. When the scores are measured on an interval or ratio scale, the standard deviation is the most used statistic. The standard deviation defines variability as the typical deviation from the mean of the distribution. Just as deviation scores helped you understand how the mean defines center, deviation scores also describe how the standard deviation defines variability