Hierarchy of Propositions
Definition
A framework that classifies forensic questions at source level (where did this come from?), activity level (what was the person doing?), and offence level (was a crime committed?). Each level requires more inference and carries more uncertainty.
- Source level
- Where did this material come from?
- Activity level
- What was the person doing?
- Offence level
- Was a crime committed?
- Trend
- Inference and uncertainty increase at each higher level
Common questions
Why do forensic scientists avoid giving opinions at the offence level?+
The offence level requires legal judgment about guilt, which is the trier of fact's role, so scientists are generally expected to address propositions no higher than the activity level and let the court draw the final offence-level conclusion from the totality of evidence.
Can the same DNA finding support both a source-level and an activity-level proposition?+
The underlying result can feed both, but each level needs its own evaluation. A source-level match tells you whose DNA it is, while an activity-level opinion about how it got there needs additional data such as transfer and persistence studies, so the two are assessed separately, not automatically together.
Related terms
- Activity-Level Interpretation
- An opinion about what activity produced or deposited the evidence, rather than simply what the source of the evidence is. Activity-level conclusions...
- Bayesian Reasoning
- A framework for updating beliefs in the light of evidence. Prior probabilities are combined with the likelihood of the evidence to produce...
- Hypothesis
- A proposed explanation for observed evidence. In reconstruction, competing hypotheses are evaluated against the physical facts and background knowledge. A good hypothesis...
- 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...
- Reconstruction
- The stage at which all analytical findings are assembled into a coherent account of how an event unfolded. Reconstruction draws on scene...