 The validity of individual test scores may be threatened when the examinee's answers are governed by factors other than the psychological trait of interest - factors that can range from something as benign as the examinee dozing off to concerted fraud efforts.Person-fit methods are used to detect item-score vectors where such external factors may be relevant, and as a result, indicate invalid measurement.Unfortunately, person-fit statistics only tell if the set of responses is likely or unlikely, and cannot prove anything.