Chapter 3 Probability to describe uncertainty

We saw in the previous chapter that there is uncertainty in scientific evidence. By asking a few simple questions about the uncertainties in a particular context, we demonstrated a systematic approach to assessing the uncertainty which can easily be applied to forensic evidence. As part of this process, the expert assesses the magnitude of their personal uncertainty given the scientific expertise and experience that they have, e.g. out of every 1000 similar case circumstances, the expert believes 200 would yield a fibre match if the suspect had truly been at the crime scene. This magnitude is based upon a probability assignment by the expert. In this chapter we discuss probability, describing what it is and using examples to demonstrate some of its useful properties as a framework for handling uncertainty.