Answer to Question #219831 in English for Rudzani

Question

Discuss the theory of investigative principles of identification and individualisation

Answer 1

Forensic scientists are sensitive to the breadth of the word ‘identification’. It can mean classification, as in ‘I identified the specimen as an 'Ursus horribilis’ or ‘an automobile as a red Buick’. It also can mean ‘individualization’, as when a fingerprint analyst testifies that ‘I identified the latent fingerprint as having been made from the right ring finger of the defendant’. In the latter situation, ‘the term “individualized” would be more felicitous’. But the term ‘individualization’ admits further distinctions. Does the individualization mean that, no matter how many people could be considered as possible suspects, the analyst confidently can state that the defendant is the source of the latent print? Or does it mean that with respect to a smaller set of possible perpetrators, the defendant is far and away the most probable source?

To highlight this distinction, we can call the former identification an instance of universal individualization and the latter an assertion of local individualization. As noted above, forensic scientists distinguish between ‘class characteristics’ and ‘individual characteristics’ but all identifications are classifications. Some of the classes are simply larger than others. The larger the class, the less discriminating the identification, but all such associations provide relevant information. A Bayesian perspective brings these two ideas together. Whether the individual is the source of the trace evidence depends on the prior odds and the likelihood ratio. The prior odds are influenced by the size of the plausible population of suspects.