A Study Of Uncertainty Evaluation Paths For Trace Evidence In A Likelihood Ratio Perspective

In today’s increasingly complex scientific evidence and at a time when uncertainty is prevalent,the question of how to clarify the sources of uncertainty,quantify the degree of uncertainty and report uncertainty in opinions is one that forensic scientists,evidence jurists and fact finders need to consider together.As a type of scientific evidence,trace evidence,although produced in the laboratory by relying on high-precision instrumentation,cannot eliminate the inherent unreliability of scientific evidence and the uncertainty factor unique to trace evidence.The traditional pathway of trace evidence testing and identification only describes the test results,rather than evaluating them,and fails to effectively evaluate uncertainty.Firstly,most of the traditional paths only consider the degree of similarity between features,and not enough attention to the level of rarity of features,making the results less objective;secondly,the forensic scientist at the request of the commissioning authority or party to conduct the test,the interpretation of the test results only reflect the position of one party,the lack of alternative interpretation of the results by the suspect’s side,resulting in a lack of balance;in addition,the traditional route to an expert opinion is often formulated a posteriori,making the opinion illogical;the reports generated,which also fail to quantify the probative value of the evidence,are unclear as to how far the test results support a party’s claims and are less readable for judges and lawyers,which makes them less transparent.The Likelihood Ratio(LR)method,which is regarded as a standard paradigm in forensic science,is a method for objectively evaluating the value of evidence by choosing probability as a measure of uncertainty and is able to objectively quantify the degree of uncertainty.This study aims to explore a method for evaluating the uncertainty of trace evidence based on the LR,and is divided into four parts: the first part is the origin of the problem,through analysing the uncertainty of trace evidence,including the nature of uncertainty and its multiple sources,and discussing the shortcomings of the traditional trace evidence testing path in evaluating uncertainty.The second part is the construction of the pathway,which firstly defines the basic concepts of evidence evaluation,explains the basic principles of evidence evaluation in the field of court science,and outlines the basic framework of the LR method,based on which the pathway of evaluating trace evidence based on the LR is constructed,including the five links of claim making,data collection and processing,model construction,model validation,and report conclusion,and discusses the research content of each link.The fourth part is the application dilemma and outlook,which analyses the technical difficulties and institutional dilemmas that may exist in the promotion of the LR method in the field of trace evidence in China from both technical and legal aspects,and attempts to propose feasible suggestions and solutions.In summary,this paper takes the uncertainty of trace physical evidence as the origin of the problem,and the likelihood ratio-based method of evaluating trace physical evidence as the path of the problem,and explores the path construction,method application,characteristics and advantages of the LR method,as well as the dilemma of the application of the method in the present day,and looks into the future.The research presented in this paper fills a gap in the relevant field and is of some reference value to the work of forensic scientists in China in examining trace evidence and how judges and other fact finders can objectively examine the value of trace evidence.