1. a deviation from true or accurate information (e.g., a wrong response, a mistaken belief). 2. in experimentation, any change in a dependent variable not attributable to the manipulation of an independent variable. 3. in statistics, a deviation of an observed score from a true score, where the true score is often defined by the mean of the particular group or condition in which the score being assessed for error occurs, or from the score predicted by a model. Errors generally are categorized as systematic error or random error. See also residual.