# Normal Distribution

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A term used to describe a set of data, that when plotted, forms the shape of a symmetrical, bell-shaped curve. id="footnote162a"> class="nounder totri-footnote" href="https://www.safaribooksonline.com/library/view/universal-principles-of/9781592535873/xhtml/ch77_fn.html#footnote162">1

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.

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. However, it is a mistake to conclude that the average is the preferred design parameter because it is the most common. Generally, a range across the normal distribution must be considered in defining design parameters, since variance between the average and the rest of the population translates to the variance the design must accommodate. For example, a shoe designed for the average of a population would fit only about 68 percent of the population.

Additionally, it is important to avoid trying to create something that is average in all dimensions. A person average in one measure will not be average in other measures. The probability that a person will match the average of their population group in two measures is approximately 7 percent; this falls to less than 1 percent for eight measures. The common belief that average people exist and are the standard to which designers should design is called the “average person fallacy.” id="footnote163a"> class="nounder totri-footnote" href="https://www.safaribooksonline.com/library/view/universal-principles-of/9781592535873/xhtml/ch77_fn.html#footnote163">2

Where possible, create designs that will accommodate 98 percent of the population; namely, the first to the 99th percentile. While design considerations can be expanded to accommodate a larger portion of the population, generally, the larger the audience accommodated, the greater the costs. Consideration of the target population is key. When designing specifically for a narrow portion of the population (e.g., airline seats that will accommodate 98 percent of American males), it is crucial to obtain the appropriate measurement data for this very specific group.