Bayesian Likelihood Ratio
Definition
A statistical framework for expressing the evidential value of a comparison: the ratio of the probability of the observed RI similarity given common source to the probability given different sources; used in advanced glass-comparison reports in the Netherlands, UK, and increasingly other jurisdictions.
- Field
- Glass comparison, refractive index evidence
- Measures
- Probability of RI similarity under same-source vs. different-source
- Jurisdictions using it
- Netherlands, UK, and increasingly others
- Underlying data
- Refractive index measured by GRIM/Mettler hot stage
Common questions
Why report a likelihood ratio instead of just saying two glass fragments match?+
A bare match statement does not convey how common that RI value is in the general population of glass, so a likelihood ratio expresses the evidential strength by comparing how likely the observed similarity is if the fragments share a source against how likely it is if they do not, giving the court a properly calibrated statement.
What background data does a valid glass likelihood ratio depend on?+
It depends on a representative database of RI values across relevant glass populations, such as vehicle glass or float glass, since the ratio is only as sound as the frequency data used to estimate how likely the observed similarity is by chance.
Related terms
- 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,...
- Becke Line
- The bright optical halo at the edge of a solid fragment immersed in a liquid, visible under the microscope; moves toward the...
- Cauchy Equation
- The empirical equation n(λ) = A + B/λ² + C/λ⁴ that describes the wavelength dependence (dispersion) of a material's refractive index; used...
- Dispersion
- The variation of refractive index with wavelength; all glasses and immersion oils show dispersion, which is why RI must be specified at...
- 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...
- 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...
- Immersion Method
- The forensic technique of matching a glass fragment's RI to that of a reference oil by adjusting temperature; the fragment becomes invisible...
- Locke 1985 Protocol
- The calibrated immersion-oil series and operational procedure for forensic glass RI measurement described by Locke and Underhill (1985); specifies the oil series,...
- Mettler FP-90
- A general-purpose laboratory hot stage widely used in forensic glass RI measurement to provide ±0.1 °C temperature control for the manual Becke-line...
- Phase-Contrast Microscopy
- An optical technique that converts phase differences (RI differences at an interface) into intensity differences visible in the image; exploited by the...
- Refractive Index (RI)
- A physical property of glass measuring how much light bends as it passes through. RI is the primary discriminating characteristic used in...