5.15 Communication

The LR needs to be successfully communicated to fact-finders and legal counsel, as well as effectively communicated by expert witnesses, legal counsel, and the judge if it is the subject of any clarifications or guidance to jurors for a particular case. The expert determines the LR, but accurately communicating its meaning is a shared responsibility.

There are currently two prominent strategies for communicating LRs as an expert: numerically and verbally. In expert reports, the currently endorsed approach by the European Network for Forensic Science Institutes is to communicate LRs using their numerical value possibly converted into the verbal statements given in Table 5.3.

Suppose the following evidence and competing propositions:

  • \(E\): Blood-spatter pattern on the defendant’s jeans. The blood is thought to originate from the complainant, which is a claim supported by DNA analysis, and this is not disputed by the defence.
  • \(H_p\): the defendant kicked the complainant,
  • \(H_d\): the defendant did not kick the complainant. They claim to have been present but standing at least 5 metres away from the attack.

Suppose also that an expert has assigned an LR of 1000 in favour of \(H_p\) compared to \(H_d\). The most raw form of communicating this LR is in its numerical format. Such a statement could read like this:

This pattern of blood spatter is 1000 times more likely to be seen if the defendant kicked the complainant, rather than if the defendant did not kick the complainant but was present during the attack standing at least 5 metres away.

Converting an LR of 1000 into a verbal statement using Table 5.3 could lead to a statement such as:

This pattern of blood-staining provides strong support for the defendant having kicked the complainant, rather than the defendant not having kicked the complainant but having been present during the attack standing at least 5 metres away.

Which of these statements is selected is a decision made by the expert or is determined by their employer’s operating procedures. In any case, a justification for the LR being 1000 should be made clear in the expert’s report. If any data have been used then this should be detailed. Any expert knowledge and experience should also be detailed for audit. For example, an LR of 1000 may have been assigned as a conservative value based upon findings in academic literature and internal experiments - details of which could be disclosed in the report. The robustness of this LR is more transparent with this information.

Suppose now that the expert had not detailed an LR of 1000 and instead opted to communicate the probative value using only the corresponding verbal category of ‘strong support.’ This could mean one of two things:

  1. An LR has been assigned to a single value within the range of the verbal category and this was not communicated,
  2. The LR has been assigned to an interval covered by the verbal category and this was not communicated.

The reasons for the expert to choose either of the above options could be useful to assess the robustness of this LR. Using an interval for the LR represents uncertainty that may or may not have been accounted for when assigning a single value. On the other hand, the single value may have been a conservative assignment.

The effectiveness of communicating the LR is an active area of scientific research. Other methods of communication are likely to develop as we gain a better understanding of how LR communications are received and actioned upon by those within the courtroom.