Source-Level Proposition
Definition
A proposition about the origin of material. Example: 'The DNA profile originated from the suspect' versus 'The DNA profile originated from an unknown person.' Source-level propositions are often not the most legally relevant question, which is usually how the material came to be where it was found.
- Concerns
- Origin of material, e.g. did the DNA come from the suspect
- Example pair
- Profile originated from the suspect versus from an unknown person
- Limitation
- Often not the most legally relevant question
- Field
- Evaluative forensic report writing
Common questions
If source-level propositions are often not the most legally relevant question, what usually is?+
The more relevant question is typically activity-level, how the material came to be where it was found, for instance whether contact occurred during an alleged assault, since a court is usually more interested in the circumstances of deposit than in origin alone, and reports should address the level actually needed by the case.
Why can't a forensic examiner report at the offence level even if asked to?+
Offence-level conclusions require weighing non-forensic case circumstances, such as opportunity, motive and other evidence, that fall outside the examiner's expertise and access, so evaluative reporting frameworks confine the examiner to source or activity level propositions within their scientific competence.
Related terms
- Activity-Level Proposition
- A proposition about what happened. Example: 'The suspect handled the knife' versus 'The suspect never handled the knife.' Activity-level propositions require the...
- Analytical Agreement
- Samples were not distinguished by the specified methods and comparison criteria. Agreement and failure to discriminate must be described at the method’s...
- Likelihood Ratio
- The probability of observations under one specified proposition divided by their probability under another, conditional on the stated information. A likelihood ratio...
- Likelihood Ratio (LR)
- A ratio comparing the probability of findings under one stated proposition with their probability under another, conditional on relevant information. Its direction...
- Offence-Level Proposition
- A proposition about guilt. Example: 'The suspect committed the assault' versus 'The suspect did not commit the assault.' Forensic scientists must not...
- Prosecutor's Fallacy
- The error of treating the RMP (or its reciprocal) as the probability that the defendant is innocent, or as the probability that...
- Standard Error
- Uncertainty measure for an estimate under a sampling model, distinct from sample standard deviation. It differs from the spread of individual observations...
- Verbal Equivalent Scale
- A standardised mapping from LR ranges to descriptive phrases, such as the ENFSI scale: LR 10 to 100 corresponds to 'moderate support',...
Explained in these topics
- Glass Comparison: RI, Density and LA-ICP-MSProposition about the material source rather than the activity causing transfer.
- Writing Evaluative Statements in Forensic Reports