Methodological Approaches to Calculating Key Validation Parameters of Forensic Methods

Validation of forensic methods (FMs) is one of the main procedures of standardization of forensic activities aimed at verifying the reliability of the results. This procedure is extensively used in organizations of the European Network of Forensic Science Institutes (ENFSI), which includes the Russian Federal Centre of Forensic Science of the Ministry of Justice of the Russian Federation (RFCFS). In terms of metrology, FMs can be divided into two types: forensic measurement methods (FMMs) and forensic testing methods (FTMs). In an earlier paper, the authors have shown that methodological approaches to FMM validation are well developed and are actively used in RFCFS laboratories, but FTM validation procedures are still a very questionable matter of extensive discussion in scientific literature. The most significant difficulties in FT validation are related to selecting validation parameters, developing the validation experiment, and performing statistical calculations. This article proposes methodological approaches to statistical assessment of FMM and FTM parameters that can be used in forensic practice. A number of recommendations for the validation procedure, a list of validation parameters, and some designs of specific experiments of FMM and FTM quality assessment are also provided. Fitness of FMMs is assessed by repeatedly measuring a monitored indicator in reference samples and standard additions using standard formulas for calculating statistical parameters. The FTM validation procedure is considered by the example of the FTM “Microscopic Examination of Textile Fibers” in which the test samples were fibers from the laboratory collection with known tested characteristics. It is demonstrated that, when assessing the reliability of FTMs and the competence of experts, it is efficient to use probabilistic estimates of the rate of false test results and to calculate the likelihood ratio.