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.

This work is part of a series of efforts towards validation of methods used in forensic fiber analysis. These efforts address current needs for accreditation of forensic laboratories and quality control in operations.The qualitative testing methodology consists of obtaining absorption spectra with the microscope spectrophotometer MSFU-K and comparing the spectral characteristics of color in fiber samples. The expert determines whether the textile fibers submitted for analysis match in color or not, depending on the results of spectral comparison.The proposed validation experiment algorithm is designed for evaluating uncertainty in optical density measurements and the level of expert competence.In this case uncertainty corresponds to reproducibility standard deviation. To evaluate uncertainty, two operators took readings of absorption spectra of dyed fibers independently in the course of three days, and measured optical density at maximum and minimum absorption wavelengths. To evaluate repeatability, 5 spectra were obtained in a row on each of the three days.The testing was conducted using three samples of polyacrylonitrile (PAN) fibers. Key characteristic points in the samples’ absorption spectra covered a wide range of wavelengths in the visible spectrum. Measurements were taken using the MSFU-K microspectrophotometer, which consists of a microscope with a spectrophotometric add-on unit.Statistical analysis of measurement data demonstrated uncertainty levels between 7,1 % and 22,1 %. Uncertainty values below 30 % are indicative of quantitative measurements and insignificant variance of optical density values, which corresponds to high reproducibility of spectra and allows the expert to make statistically reliable match/non-match conclusions on the color of compared fibers.Expert competence was assessed based on «blind» test results. The experts had to determine which of the three samples were colored with the same dye. Each of the two experts was provided with 3 visually identical samples that were colored with different dyes. The experts were asked to distinguish between fibers treated with the same dye. When analyzing obtained spectra, both experts correctly identified same-color fibers based on matching color spectral characteristics.Positive validation results suggest that the MSFU-K microscope spectrophotometer can be successfully used in forensic fiber analysis for measuring the color of dyed fibers.

The article reviews and summarizes the experience of validating forensic expert techniques in the Russian Federal Centre of Forensic Science of the Ministry of Justice of the Russian Federation. The authors point out the methodological features of practical implementation of the validation procedure. They demonstrate that the specificity, diversity, and complexity of the objects of expert study require the classification of the applied methods in terms of metrology, identification of the main validation parameters of quantitative and qualitative methods, organization of experiments, and evaluation of validation parameters using mathematical analysis methods. They also propose to divide methods into two types: forensic expert measurement methods (FMT) and forensic expert testing methods (FTT). Based on the generalization of information presented in several regulatory documents and scientific publications, the following parameters are identified for FMT: metrological characteristics or properties of the method (specificity, linearity, sensitivity, range of determined values, detection limit, quantitative determination limit) and quality indicators of the method (precision, correctness, accuracy of the analysis result, or uncertainty). When validating FTT, it is proposed to evaluate the reliability of the method and the competence of the expert.An experiment to assess validation parameters is performed using enough control samples with established characteristics of controlled indicators and with the participation of a sufficient number of experts. Requirements for control samples are provided.The authors also give examples of probabilistic evaluation of validation parameters for two qualitative testing methods: microscopic examination of textile fibers and detection of gunshot residue using scanning electron microscopy and X-ray microanalysis. The reliability of these methods is assessed by calculating the likelihood ratio, and the specificity of interpreting the results of FMT and FTT validation is noted.The decision on compliance with the requirements is made if the interval of the established extended uncertainty for the obtained result does not exceed the tolerance field. In the absence of tolerances, FMT is considered suitable for solving forensic expert tasks if the values of the extended uncertainty of the measurement results of the controlled indicator do not exceed the values established during validation. For FTT, a low probabilistic proportion of false positive and false negative results in determining the presence/absence of controlled indicators, as well as experimentally confirmed competence of the expert during validation, are indicators of the suitability of the method for its intended use

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