The development of microbiological reference materials (RMs) has succesfully led to the certification of six materials (CRMs) and to the production of a wider range of non-certified reference materials. Both materials (RM and CRM) are prepared in the same way. They consist of gelatin capsules filled with milk powder artificially contaminated with a selected test strain. The difference between an RM and a CRM is the degree of control of the materials. Because of this difference the RMs and CRMs differ largely in price and application. An RM is intended to be used in daily quality control (QC) of microbiological (routine) analysis. For this purpose the use of a control chart can be very useful. A CRM is intended to be used incidentally, e.g. for validation of a (new) method, or for a complete check of the performance of the laboratory and technicians. The certified value of a microbiological CRM is method dependent. A user of a CRM will compare their mean results with the given certified value and confidence limits. With a so-called power calculation technique the optimal number of capsules and replicates for a CRM were calculated.

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