Prosecution Proposition
Definition
In the Bayesian evidence framework, the proposition that the questioned evidence item was in contact with or came from the crime scene; paired with a defence proposition for the likelihood ratio calculation.
- Context
- Soil comparison casework
- Typical content
- Item was in contact with the crime scene
- Paired with
- A defence proposition for the LR
- Framework
- Bayesian evidence evaluation
Common questions
Why does soil evidence need explicit propositions when a visual match seems obvious?+
Soil composition can vary naturally within a small area or recur at unrelated locations with similar geology, so a simple visual or mineralogical match is not informative on its own; stating propositions forces the analyst to weigh how likely that match is under an innocent alternative too.
What alternative propositions are common in soil casework?+
A defence proposition might state that the soil came from a location the defendant regularly and innocently visited, or that similar soil is common across a wide geographic area, both of which the analyst must consider when forming the likelihood ratio.
Related terms
- Bayesian Inference
- A framework for updating probability estimates in the light of new evidence; in forensic science, used to combine the prior probability of...
- Control Sample
- A water sample collected from the suspected drowning environment at or shortly after the time of body recovery. Its diatom assemblage is...
- ENFSI ENG-FG1
- The European Network of Forensic Science Institutes' guidelines for forensic geological evidence, published in 2018 by the Expert Working Group on Forensic...
- FBI Soil Examination Unit
- The specialist unit at the FBI Laboratory in Quantico, Virginia, that has conducted forensic soil comparisons in federal criminal cases since the...
- 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...
- Mahalanobis Distance
- A multivariate distance measure that expresses the separation between two observations in units of the population standard deviation, accounting for covariance; used...
- Murray-Tedrow Framework
- The tiered forensic-soil comparison protocol established by Raymond Murray and John Tedrow in their 1992 text 'Forensic Geology,' which organises soil examination...
- Principal Component Analysis (PCA)
- A multivariate statistical method used in fire debris research to reduce chromatographic data matrices to principal components that capture major variance. Used...
- Transfer and Persistence
- The processes by which soil particles move from a source to an evidence item and remain there; relevant to forensic comparison design...
- Verbal LR Scale
- A standardised conversion table from numerical likelihood ratio values to phrases such as 'moderate support' or 'very strong support' for one of...
- Within-Scene Variability
- The natural compositional variation of soil properties over short distances within a single scene; must be characterised by multiple control samples before...