One of the most important tasks of water microbiology laboratories is to provide accurate and reliable analysis results when the customer undertakes. Laboratories receive accreditation certificates by applying to various national and international accreditation bodies to demonstrate that the results of the analysis are accurate and reliable on the international platform. In Turkey, this certification is carried out by the Turkish Accreditation Agency (TÜRKAK). One of the most important subjects examined under the TÜRKAK inspection in the laboratories is the measurement uncertainty of laboratories according to TS EN ISO / IEC 17025 standard. The reproducibility results are used in the calculation of measurement uncertainties in water microbiology laboratories. In Escherichia coli measurement uncertainty calculations, 60 cases of analysis were devised for 30 studies, supposed to be performed by two analysts (Analyst and Analyst B). As a result of our studies, RSDR was found to be 0.075. RSDR was considered to be combined uncertainty. For our 95% confidence interval as a result of our studies, the measurement uncertainty was calculated 15. Accurate implementation of method verifications of water microbiology laboratories will contribute to improving customer satisfaction by improving public health as it will increase quality assuranceIntroduction One of the most important tasks of water microbiology laboratories is to provide accurate and reliable analysis results when the customer undertakes. Laboratories receive accreditation certificates by applying to various national and international accreditation bodies to demonstrate that the results of the analysis are accurate and reliable on the international platform. In Turkey, this certification is carried out by the TÜRKAK. This organization is an independent organization which is a member of the European Accreditation Association (EA), the International Accreditation Forum (IAF) and the International Laboratory Accreditation Association (ILAC). TÜRKAK conducts audits in accordance with ISO / IEC 17025 standard, which sets general conditions for testing and calibration laboratories, one of the quality management systems. One of the most important issues examined under the audit under the ISO / IEC 17025 standard is the measurement uncertainty of laboratories (1). Since standard methods are often used in water microbiology laboratories, a complete validation study is not performed. As stated in ISO / TR 13843, secondary validation, in other words, verification studies is sufficient (2). Prior to TÜRKAK's application, laboratories are responsible for proving the validity of the standard methods used by the laboratories, ie the methods for their laboratories. Method verification is the proof of the suitability of a measurement procedure for its intended use by written tests with objective testing with various performance studies The method verification also provides information on both analytical requirements (incubation temperature, food preparation and storage conditions) as well as results such as recovery (3). One of the most important criteria prior to laboratory work is the calculation of measurement uncertainty as well as starting work with competent and sufficient personnel (4). Measurement uncertainty results, which are part of the verification studies, show how accurate and reliable the analysis can be. Measurement uncertainty indicates the range of values that may be encountered with the measured value and the value supplied with the measured value. The measurement uncertainty account in microbiology laboratories is a category that prevents metrological and statistically valid computation, so it is difficult to calculate each measurement uncertainty source (5). To understand measurement uncertainty calculations in microbiology laboratories, ISO / TS 19036 (Microbiology of food and animal feeding stuffs -- Guidelines for the estimation of measurement uncertainty for quantitative determinations) is a good standard. In this standard, measurement uncertainty calculation is proposed based on experimental studies carried out in the laboratory with a black box approach that measures total variation irrespective of source (6). However, since ISO / TS 19036 is a food microbiology standard, it cannot be used in the measurement uncertainty calculations of samples in water microbiology laboratories. In water microbiology laboratories methods such as ISO / TR 13843 and ISO 17994 which provide statistical information can be used in method verification (2,7). However, laboratories may have difficulty reaching these documents because these standards have to be paid. However, the Canadian laboratory accreditation document P19 CALA provides free access (8). Another advantage of this method is that it allows the calculation of measurement uncertainty using routine samples in the laboratory. In this work, it is aimed to introduce the P19 method of the Canadian Accreditation Laboratories Association (CALA), which can be practically applied at low cost for measurement uncertainty calculations in method verification studies to accredited water microbiology laboratories according to the ISO / IEC 17025 (1).