APA Dictionary of Psychology

a score that is missing from a data set, either because the event of interest has not occurred by the end of the study period or because the response falls into an unmeasurable portion of the scale. A response measured by a meter, for instance, may not be recorded because it is too small to be detected or is so large that it exceeds the meter’s capability to record. In such a situation, the minimum value is assigned to an undetectable observation, and the maximum value is assigned to an excessive observation.