3.5 Conditioning on information

Information which is used to construct a probability is called conditioning information. This is because we are constructing a probability which is conditional on that information, otherwise known as a conditional probability. The process of using information this way to construct such a conditional probability is itself called conditioning.

This means that probability is conditional on our current state of individual knowledge. Conditioning information can be broken down into two types:

• information that is emphasised in the conditioning of the probability. This information is explicit and critical to the application of the probability. For example, the hypotheses of the prosecution and defence.
• all other information. This includes assumptions and other contextual information, otherwise known as background information. An example of this is the relevant case circumstances that are agreed by both the prosecution and defence. This information can be tedious to state each time a probability is mentioned and so it is often stated once and then only mentioned again if it changes.

The distinction between these two types of information is context-dependent since it is just a matter of emphasising different contextual information. Information that is important to emphasise in one context may be background information in another. For example, when interpreting fibre evidence recovered from a crime scene, the expert may wish to condition on the person of interest having been present at the crime scene. This would be background information in a case in which the person of interest admits to having been present at the crime scene. Otherwise it would be disputed by the person of interest and thereby become a critical piece of conditioning information.