4.11 Sub-source level propositions

The sub-source level is a particular version of the source level. It makes source attributions of more general traces, such as DNA to an individual. The difference between source and sub-source is that the source level states the specific cellular material (e.g. blood) that DNA is recovered from, whereas sub-source simply states that DNA has been found. This is the result of highly sensitive modern measurement technologies. Taking swabs of items of interest at a crime scene can detect very low levels of DNA even though there was no obvious biological material that was swabbed. The sub-source level is less-informative than the source level because low levels of DNA can easily be transferred between people and surfaces with minimal, and sometimes no, direct contact.

This distinction between source and sub-source level can be key in case circumstances because it leaves open questions of how the DNA came to be there. The sub-source level propositions in Table 4.1 are:

• \(H_p^7\): The DNA on the claimant’s clothing came from the defendant,
• \(H_d^7\): The DNA on the claimant’s clothing came from someone else.

DNA found on clothing may be explained by direct contact and indirect transfer, which might be reasonable in any given case. This means that more obstacles are encountered than source level propositions when trying to move to the activity level. If blood has been found on an item, and so the source level can be addressed, then there are far fewer mechanisms of it coming to be there compared to finding traces of DNA alone.

The hierarchy of propositions is a general conceptual framework which is helpful when considering different propositions. Individual case circumstances will determine how important each level of proposition is and the degree to which the scientist can consider them. In some cases, source level propositions may effectively address the offence level propositions based on the case circumstances and witness testimony.