Difference between the average measured mass or concentration and reference mass or concentration expressed as a fraction of reference mass or concentration. Source: NIOSH Manual of Analytical Methods. NIOSH Manual of Analytical Methods (NMAM), 4th ed. DHHS (NIOSH) Publication 94-113 (August, 1994) A systematic error inherent in a method or caused by some feature of the measurement system. Source: Exposure Factors Handbook