Statistical Analysis in Forensic Science: Evidential Values of Multivariate Physicochemical Data

Appendix A

Probability

A.1 Laws of probability

Probability is a measure of uncertainty (Lindley 1991). It can also be a subjective measure of belief. For example, in a sporting context, there are many references to the probability that a certain team will win a particular football match or a certain horse will win a particular race. Probability can also refer to events that have happened in the past if their outcome is unknown by an enquirer. Questions are asked about a personal belief about the outcome of a sporting event in the past, such as what is the probability a certain team won a particular match? Of course, the question can be answered by checking records, but until then the knowledge is uncertain and this uncertainty can be represented by a probability.

The subjectivity can be extended beyond a sporting context to a legal context (Aitken and Taroni 2004; Lucy 2005). There is uncertainty in a criminal trial. In practice, it cannot be said that it is certain that the defendant is guilty or non-guilty. The uncertainty can be measured by probability. It may be cloaked in verbal phrases, such as it is almost certain the defendant is guilty or it is clear that the defendant did not commit the crime, but these are only verbal expressions of probability. If the issue of guilt or otherwise is considered the ultimate issue, there are also intermediate issues to which probability can be applied. A match is declared between a defendant’s DNA profile and the DNA profile extracted ...