6.1 An introduction to Bayesianism

You were introduced to probabilities in Section 5 of this course. As you saw, probabilities are expressed in figures between 0 and 1, where 0 is impossible, 1 is definite, and 0.5 is evenly balanced. These can also be expressed equivalently as a percentage of 0%, 100% and 50%. A legal case will often have a lot of different probabilities that need to be combined. For example, in a murder case:

• What is the probability that an eyewitness is reliable?
• What is the probability that the accused’s alibi is fake?
• What is the probability that the blood on the murder weapon is that of the victim?
• What is the probability that the accused hasn’t been framed by someone?

Bayes’ Rule provides a mathematical means of combining these probabilities to estimate the final probability of guilt. It is quite technical, so you will not go into much detail here, but if you are very interested, links are provided in the ‘Exploring further’ section at the end of the course. Instead, you will go through a short exercise in the next section that demonstrates how Bayes’ Rule works in practice.