4.1 Due to requirements being placed on concentrations of substances within (or on) materials, assessing conformance of products has become a complex, time-consuming and expensive task. This guide is intended to assist the user in developing a protocol for product assessment.

4.2 A priori knowledge is based on logical deduction and scientific principles, so actual testing of a material may not be required in order to assess conformance to requirements. For example, it is possible to deduce that organic substances will not survive the temperatures required to produce wrought steel, so there is no need to test for organic substances in wrought steel nor is it possible to develop test methods and reference materials for determination of organic substances within wrought steel.

4.3 A posteriori knowledge is based on observation, experience and known facts. If a priori knowledge cannot rule out the possibility that a substance is present within (or on) a material, a test method may be required to verify or generate information on the concentration of that substance within (or on) the material.

4.4 Test methods can be used as a means to verify and provide information related to substances within materials. At the same time, misinformation can be generated or inappropriate conclusions drawn when test methods are misapplied. This guide is intended to provide recommendations on the application of test methods.

4.5 Test methods may be applied by producers or by interim or end users of materials. However, it is not necessary or cost effective to test materials at each stage of production. The decision to apply test methods and the frequency of testing should be based on risk perceived by the user or can be a matter of agreement.

4.6 Assessment of plastics for presence of declarable substances is more complex than assessment of metals and alloys, since the possible ingredients are comparatively much more numerous in plastic manufacture.

4.7 This guide includes a general process and case studies in order to provide guidance and to distinguish where a priori and a posteriori knowledge should be applied. Flow charts as a guide for assessment of materials and products are provided in Appendix X1.