4.10 Source level propositions

Source level propositions relate to the origin of traces and impressions found, e.g. at a crime scene. The source level makes attributions such as blood to an individual, fibres to a garment of clothing, footwear marks to a piece of footwear, etc. These propositions are relatively simple compared to the activity level since they do not comment on questions such as “how?” or “why?” the traces and impressions may have come to be where they were recovered from. The answers to questions of activity are often the more consequential than questions of source because they are logically closer to questions about offences. For this reason, the source level is placed below the activity level in the hierarchy.

Consider the competing propositions from Table 4.1:

• \(H_p^5\): 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.

These propositions have been derived from evidence that has been found. This is a common situation for source level propositions: trace evidence is found and we need to know its origin. It is also very common for forensic scientists to consider source level propositions. This is because the science is well-equipped to address them. For \(H_p^5\) above, the scientist can compare analytical measurements from the recovered glass fragments to other fragments known to come from the smashed window. For \(H_d^5\), the scientist will need to compare analytic measurements of the recovered fragments to fragments from other known sources.

Although source level propositions are below the activity level in the hierarchy, this is not to say that they are unimportant. In fact, in individual cases, they might effectively address the most important activity level propositions. For example, if the fact-finder decides that \(H_p^5\) is true based on the expert’s evidence, then they may not require further evidence to think that the following activity level proposition is true:

• \(H_p^{5+}\): the defendant was present when the glass was smashed.

In other words, the truth of the source level proposition could have a logical domino effect that leads to the fact-finder being sure of the truth of important activity level propositions.