If we were to sample a population of high school seniors on hand grip strength, we would probably get a distribution that looks like the following:

Frequency Distribution Diagram showing a bi-modal distribution indicating two separate underlying populations.

This distribution is typical of a "bi-modal" distribution and indicates there are really two separate underlying populations and not one.  In this instance, the two populations are: male and female with different distributions for each.  As many statistical tests are valid only for normal (uni-modal) distributions, it is important to test our data distributions before proceeding further with statistical assessment and analysis or risk erroneous conclusions.

Understanding individual differences and the distribution of human variability through rigorous statistical testing are "core" concepts within Human Factors.  To illustrate how these concepts are applied, we will next address anthropometrics.