**Normal distribution**refers to a frequency distribution, defined by a particular mathematical function that is bell shaped, is unimodal, is symmetrical, and has the same mean, median, and mode; a symmetrical, bell-shaped curve that describes the variability of certain characteristics within a population; most people fall at or near the average score, with relatively few at the extremes of the distribution. Normal distribution, moreover, is a frequency distribution in which the values or scores group around a mean. In neuropsychological testing , many test scores display such distributions.