Quality assurance in a testing laboratory, particularly in the case of its assessment (see ISO/IEC Guide 25), highlights the need to consider closely the question of the accuracy of its measurements and analytical results, and to ensure that the principles necessary to establish demonstrated accuracy have not been omitted. The calibration of the parameters associated with chemical analyses and material testing deserves particular attention, because major errors can be made by neglecting or ignoring the basic principles of metrology which also apply to these areas. This Guide identifies a number of general recommendations for those who, either in laboratories or as assessors, are faced with this problem.