Random variation is the term given when a process does not exactly meet specifications consistently. It is always uncontrollable. However, by controlling nonrandom variation, and determining process stability, we can increase the chances of processes meeting specifications.

For a production line, random variation could take the form of material purity, size, performance specifications. Controlling random variation would require developing tests to measure the quality and attributes of the composite parts before assembling them.

For a service-oriented business, random variation is inherent in practically every request. For a landscaper, weather is a random variation. Scheduling the mowing of lawns before it rains would be a method to accommodate for the variation, rather than to control such a variation as weather.