* Definition of bias - distinct from accuracy, bias is an average deviation from a true value. * Method comparison - a set of specimens is assayed by both an existing method and the new candidate method, and the results compared. The following list describes the testing procedures and data handling required in a method comparison study for the assessment of bias: test material; number and disposition of specimens; summary of findings; the problem with correlation, and the difference plot; statistics of difference; log transformation of the difference plot; statistics of difference with logs; linear regression; Deming and Passing-Bablok models; the value of r in linear regression; choice of statistics; examples of suitable computer programs. * Acceptable bias criteria are discussed. * Linearity and recovery - failing either of these criteria should serve as a warning that method comparison data may conceal an unrecognised bias. * Finally, consideration of all steps in the assessment of bias is required to determine acceptability or not of the method comparison.