Metrics are the generic solution for the comparison process irrespective of the nature of the trace (i.e. visual, physical, chemical or even digital).
A metric is defined as ‘‘a transformation that adds a new layer of information since it starts with entities (i.e. profiles) and concludes with a measurement describing the degree of relationship between entities (i.e. scores)’’ [4].
They are used to compare and measure the (dis)similarity between specimens. They have the critical advantage of relying on explicit, transparent and verifiable rules. They can be used with both quantitative and qualitative data, as demonstrated in previous work [15,16]. Furthermore, metrics can be seamlessly integrated with additional statistical methods commonly used to manage and process big datasets typically S. Baechler et al. / Forensic Science International 250 (2015) 44–52 45 encountered in forensic intelligence (e.g. principal component analysis).