The real number of viable cells in a sample is generally unknown. In order to get a real reflection of the number of bacteria, impacting factors must be taken into consideration and must be kept comparable. Important preconditions for the assessment and comparison of quantitative microbiological data are: Samples should be taken and prepared under circumstances as identical as possible (stage of sampling and quality of the material), structure of the laboratory technique must be comparable (preparation of the first dilution, technique of plating, media and incubation circumstances), the personal factor must remain under control as far as possible (performance of the analyses in daily work, skills of all technicians).