4.15 Changing propositions
Changes in propositions change probative value. New information may be uncovered as part of an ongoing investigation. Propositions used to determine probative value must change to include the updated information. For this reason, there is usually a caveat statement added to expert witness reports which makes it clear that any of the expert’s conclusions may change in light of new information.
One example of this is if the defendant changes their version of events or reveals previously undisclosed information. The source level glass propositions
- \(H_p^5\): The glass from the defendant’s clothing originated from the smashed window,
- \(H_d^5\): The glass from the defendant’s clothing originated from some other source,
might be the default ones considered by the scientist with a lack of other relevant contextual information. However, if the suspect states in an interview that they have only worn the same clothing recently once before whilst working on a construction site replacing windows, then \(H_d^5\) could be revised to reflect this. The defence proposition might change to:
- \(H_d^{5^*}\): The glass from the defendant’s clothing originated from the windows on the construction site.
This is a small alteration but it could diminish the probative value of the glass match. If the glass type is very rare and only used for particular housing windows, then a glass match assuming \(H_d^5\) is unlikely. However, if this glass type is used in the construction site windows from \(H_d^{5^*}\) then a match is highly likely. The evidence now has no probative value with the new contextual information and defence proposition.
Similarly, changing the level in the hierarchy of propositions changes the probative value of the evidence. Consider again the related source and activity level competing propositions from Table 4.1:
- \(H_p^4\): The defendant kicked the claimant,
- \(H_d^4\): The defendant was not present when the claimant was kicked,
- \(H_p^6\): The blood on the defendant’s shoes came from the claimant,
- \(H_d^6\): The blood on the defendant’s shoes came from someone else.
Consider the probative value of a match between the claimant’s DNA profile and the blood found on the defendant’s shoes. The probability of this match conditioned on \(H_p^6\) is much higher than the probability of the match conditioned on \(H_d^6\). This makes the evidence highly probative at the source level.
However, it is of more interest to the court to determine whether the defendant kicked the claimant or not using \(H_p^4\)/\(H_d^4\). Depending on the case circumstances, the evidence of a match may be of little value when considering the activities in \(H_p^4\)/\(H_d^4\). For example, the defendant may claim to have helped the claimant after noticing them injured in the street. The defendant claims that the blood must have transferred to their shoes whilst they were helping and so this becomes the new defence proposition. The probability of this evidence under the new defence proposition may be very high and this leads to evidence whose probative value is low at the activity level even though it is high at the source level. Other analysis might be commissioned to explore the probative value at the activity level further.
Evidence which is highly probative at the source level may have its value diminished at the activity level. For this reason, it is important to have clear and unambiguous propositions which are addressed at the appropriate proposition level (usually activity).