random variation
differences in a dependent variable that are due to chance rather than to the factors being studied. Causes of random variation in test results may include respondent factors, such as health, motivation, attention, concentration, and fatigue; situational factors, such as room temperature, noise, and working environment; and respondent-by-situation factors, such as a respondent not being prepared for a specific rating task. Researchers try to estimate the extent to which random variation may be involved in the study to understand the true impact of the factors being assessed. See also chance difference.