the difference between a teacher's obtained score and his/her true score on an assessment that is due to factors beyond the control of that teacher, including lack of reliability in the assessment instrument or process, variability of settings of the assessment, limited sampling of teacher performance, bias of the assessor, rater effects, and interactions among such factors. See Assessor, Bias, Contamination, Obtained Score, Rater Effect, Reliability, Sampling of Performance, True Score.