Methodological approaches to calculation of the main validation parameters of forensic techniques

Validation of the forensic methods (FM) is one of the main procedures for the standardization of forensic activities used to verify the reliability of the obtained results of the studies. This procedure is widely adopted in the organizations of the European Network of Forensic Science Institutes (ENFSI), which also includes the Russian Federal Centre of Forensic Science of the Ministry of Justice of the Russian Federation (RFCFS). Two types of FM can be distinguished in metrological aspects: forensic method of measurements (FMM) and forensic methods of testing (FMT). We have shown earlier that unlike the developed methodological approaches to the FMM validation which gained practical application in the RFCFS laboratories, the FMT validation procedures give rise to many questions which are widely discussed in the scientific literature. The most significant difficulties in FM validation are attributed to selection of the validation parameters, development of the validation experiment and performance of statistical calculations. We propose methodological approaches to the statistical assessment of the FMM and FMT parameters to be used in practice of forensic experts. Moreover, we present a number of recommendations on the validation procedure, list of the validation parameters, and consider specific schemes of the experiments to be used for assessing of the quality indicators of FMM and FMT. The results of multiple determination of the controlled index in the referenced samples and standard additives are presented to assess the suitability of the FMM using common formulas for calculation of the statistical parameters. Case study of the FMT validation procedure «Microscopic Examination of Textile Fibers» is considered. The expediency of using probabilistic estimate of the share of false test results, as well as calculation of the likelihood ratio in assessing the reliability of forensic methods of testing and competence of the experts is demonstrated.