Bayesian Approach/Likelihood Ratio Approach

This approach used for interpreting evidence is based on the mathematical theorem of Reverend Thomas Bayes, stating the posterior odds are equal to the prior odds multiplied the likelihood ratio (LR). If using prior odds, one will speak of the Bayesian approach, and if using only the LR, one will speak of the likelihood ratio approach.

In forensic science, the weight of evidence E (DNA, glass, fingerprints, etc.) is often assessed, using the ratio of two probabilities estimated given by two propositions (i.e., LR). One hypothesis is suggested by the prosecution (H _p) and the other by the defence (H _d). Two propositions could be for example “The blood recovered from the crime scene comes from the suspect” versus “The blood recovered from the crime scene does not come from the suspect, but from someone else, unrelated to him.” The likelihood ratio (LR) is therefore constructed as the ratio of the two probabilities of the observations given in each proposition. It can take any value...