2.1 What we mean by uncertainty

In this book, we frame discussions about uncertainty around beliefs, since beliefs about facts are the focus of forensic science. Certainty describes a belief which can be guaranteed without any doubt. If there is any doubt in the belief then it cannot be called certain. We call those beliefs uncertain. Beliefs can have varying degrees of doubt and so uncertainty can be described on a spectrum with one end containing beliefs with high degrees of uncertainty and the other containing beliefs with low degrees of uncertainty. Many beliefs come with at least some degree of doubt, however small it may be, and so must be uncertain. This makes uncertainty unavoidable.

Communicating uncertainty is key to forming and calibrating realistic expectations. When uncertainty is communicated well and everyone has realistic expectations then it can be managed effectively in order to mitigate negative consequences. However, in practice there are understandable incentives against doing this. One example is that people in positions of authority (such as politicians) do not want to diminish the trust of those they have a responsibility towards by admitting that they do not know something. Another is when the uncertainty might cause a disproportionately negative reaction, e.g. uncertainty about nuclear war. This means that unfortunately, in many aspects of life, uncertainty is simply ignored or concealed.

In society, there are many situations in which we can communicate and manage uncertainty better. This will come with building a better understanding of uncertainty and having honest and open discussions about it. We aim to contribute to that in forensic science, by framing scientific evidence evaluation around the fact that it is uncertain. In order to do this, it is useful to be more specific about different types of uncertainty.