In research and experimental situations, the term systematic variation generally denotes an anomaly or inaccuracy in observations which are the result of factors which are not under statistical control.
An example of this could in testing water samples for harmful bacteria — having no control over the vitamin/mineral content that might be present and it having an effect of the bacteria that skews the results. In psychological or social science research studies, variations can occur that are the results of differences in participants’ intelligence, education, past experiences, mood, and other variables. Random assignment is often used in order to try to minimize the risk of systematic variation.