Activity-Level Proposition
Definition
A proposition about what happened. Example: 'The suspect handled the knife' versus 'The suspect never handled the knife.' Activity-level propositions require the scientist to incorporate assumptions about transfer, persistence, and background rates, making them more complex but more directly relevant to the legal issue.
- Concerns
- What happened, not just source of material
- Requires
- Assumptions about transfer, persistence, background
- Example pair
- Suspect handled the knife versus did not
- Framework position
- Above source-level in the hierarchy of propositions
Common questions
Why are activity-level propositions considered more legally relevant than source-level ones?+
Source-level propositions only address whose cell material is present, while activity-level propositions address the actual disputed event, such as whether contact occurred, which is what the court usually needs to decide, making them more directly useful despite being harder to evaluate.
What extra evidence does an activity-level evaluation require compared to a source-level one?+
The scientist needs data or literature on transfer and persistence rates, background prevalence of the material in similar contexts, and the case circumstances, all of which introduce additional assumptions beyond simply matching a genetic profile.
Can a scientist give an activity-level opinion without transfer and persistence data for the specific scenario?+
Generally no, or only with significant caveats. Without relevant transfer and persistence studies for the type of contact and surface involved, the scientist typically must limit the opinion to source level or clearly state the assumptions and their evidentiary basis.
Related terms
- 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...
- Daubert Standard
- The US federal evidentiary standard (Daubert v. Merrell Dow Pharmaceuticals, 1993) requiring that expert testimony be based on scientifically valid methods with...
- Defender's Fallacy
- Dismissing match evidence by citing the large absolute number of people who share the characteristic, ignoring all other evidence that narrows the...
- Frye Standard
- The US legal test for admissibility of scientific evidence, originating from Frye v. United States (1923), which required that a technique be...
- Likelihood Ratio (LR)
- The ratio of two conditional probabilities: the probability of the observed evidence given the prosecution's hypothesis (same source), divided by the probability...
- 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...
- Source Attribution
- The inferential step of linking a trace to its donor. Serology identifies blood as human and potentially as a specific genetic type;...
- Source-Level Proposition
- A proposition about the origin of material. Example: 'The DNA profile originated from the suspect' versus 'The DNA profile originated from an...
- 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
- Serology Evidence in Court: Interpretation and TestimonyA proposition addressing how biological material came to be at a location, such as whether the suspect physically handled an object, as opposed to a source-lev...
- Writing Evaluative Statements in Forensic Reports