Universal Principles of Design, Updated and Expanded Third Edition

124 Normal Distribution

A bell-shaped curve formed by plotting the frequency of a variable within a population.

Normal distributions result when many independently measured values of a variable are plotted. The resulting bell-shaped curve is symmetrical, rising from a small number of cases at both extremes to a large number of cases in the middle. Normal distributions are found everywhere — annual temperature averages, stock market fluctuations, student test scores — and are thus commonly used to determine the parameters of a design.1

In a normal distribution, the average of the variable measured is also the most common. As the variable deviates from this average, its frequency diminishes in accordance with the area under the curve. It is a ...