Bayesian Likelihood Ratio (LR)
Definition
The statistical framework that evaluates glass-fragment evidence by comparing the probability of the observed fragment population assuming contact with the source glass against the probability assuming only background glass sources; the formal approach required by ENFSI ENG3-2013 and advocated by the UK Forensic Science Regulator.
- What it measures
- Probability of evidence under prosecution hypothesis divided by probability under defence hypothesis
- Glass evidence application
- Evaluates fragment populations by comparing the probability of the observed fragment population assuming contact with the source glass against the probability assuming only background glass sources
- Key standards
- Required by ENFSI ENG3-2013 and advocated by the UK Forensic Science Regulator
Common questions
What does a likelihood ratio tell you in forensic glass analysis?+
A likelihood ratio is a number that expresses how much more likely your evidence is under the prosecution's theory (the glass came from the same source) versus the defence theory (it came from different sources). The higher the number, the stronger the evidence supports a common origin.
How do forensic labs calculate the likelihood ratio for glass fragments?+
Labs measure glass fragments and compare the probability of observing those measurements if the glass came from the same source, then divide by the probability of observing the same measurements from random background glass. This statistical framework turns lab measurements into a formal strength-of-evidence statement.
Are likelihood ratios required for glass evidence reporting?+
ENFSI ENG3-2013 requires this formal statistical framework for evaluating and reporting glass fragment evidence, and the UK Forensic Science Regulator strongly advocates its use. Whether a court mandates the approach depends on jurisdiction.
Related terms
- Secondary Transfer
- The movement of a glass fragment from a primary surface (which contacted the broken glass) to a secondary surface (which contacted the...
- 3R Rule
- The fracture-analysis mnemonic: Radial cracks form at Right angles to the Reverse (far, non-impact) side of the glass. Applied to the fracture...
- ASTM E1967
- The ASTM International standard for forensic glass RI measurement; specifies three acceptable methods (automated, semi-automated, manual), minimum precision of ±0.0001 RI units,...
- ASTM E2927
- Standard test method for the determination of trace elements in glass samples using LA-ICP-MS; specifies the element suite, calibration standards (NIST SRM...
- Concentric Cracks
- Approximately circular cracks centred on the impact point; they form second, on the near (impact) face, and terminate when they encounter pre-existing...
- Cone Fracture
- A conical crack produced by concentrated contact loading (Hertzian contact), opening on the far (exit) face of the glass; the cone geometry...
- ENFSI ENG3-2013
- The ENFSI Glass Expert Working Group guideline for glass comparison casework; specifies the examination sequence, within-source variation requirements, statistical comparison framework, and...
- Entrance Bevelling
- The widening of a bullet-penetration hole toward the exit side, analogous to bevelling in bone wounds; the narrower opening is on the...
- Fracture Termination Rule
- The principle that a propagating crack terminates when it encounters a free surface created by a pre-existing crack; at any T-junction, the...
- GRIM-3
- Glass Refractive Index Measurement, third-generation instrument by Foster + Freeman (UK); an automated phase-contrast hot-stage system that identifies the RI match point...
- Hackle Marks
- Ridges on the fracture surface oriented perpendicular to the crack propagation direction; they appear in the mist and hackle zones at higher...
- Hotelling's T2
- The multivariate generalisation of the t-test used to compare mean element-concentration vectors between questioned and known glass populations; accounts for the full...
Explained in these topics
- Glass Comparison: RI, Density and LA-ICP-MSThe probability of the observed evidence given the prosecution hypothesis (common origin) divided by its probability given the defence hypothesis (different or...
- Glass Fragmentation and Direction-of-Force AnalysisThe statistical framework that evaluates glass-fragment evidence by comparing the probability of the observed fragment population assuming contact with the sou...