Within-Source Variation
Definition
The natural variation observed among multiple samples collected from a single source location. Used to estimate the probability of the observed difference between questioned and reference samples under the same-source hypothesis.
- Measured properties
- Refractive index and elemental composition
- Why it matters
- Allows valid statistical interpretation of between-source comparisons
Common questions
What is within-source variation in glass analysis?+
Within-source variation is the natural differences in refractive index or elemental composition found at different locations within a single glass pane. Forensic examiners measure this variation to understand how much scatter to expect from fragments of the same source.
Why do forensic scientists measure within-source variation?+
This measurement establishes a baseline of expected variation. Once you know how much a single pane naturally varies internally, you can compare questioned glass fragments to reference samples and interpret the results statistically. Without this baseline, you cannot tell if differences indicate a different source or just normal variation from the same pane.
How does within-source variation affect glass comparison?+
When comparing a questioned fragment to a reference sample, the observed difference could mean they are from different sources or from the same source with natural variation. Characterizing within-source variation helps estimate the probability that the difference occurred by chance, supporting likelihood ratio calculations.
Related terms
- 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...
- 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...
- LA-ICP-MS
- Laser ablation inductively coupled plasma mass spectrometry; a technique that fires a laser pulse at a sample surface, vaporises a tiny amount,...
- 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,...
- Bayesian Likelihood Ratio (LR)
- The statistical framework that evaluates glass-fragment evidence by comparing the probability of the observed fragment population assuming contact with the source glass...
- Between-Source Variation
- The variation observed among samples from different locations in the reference population. Used to estimate the probability of the observed similarity under...
- Denominator Hypothesis
- The defence hypothesis under which the LR denominator is calculated. Typically: the questioned sample comes from some other location within the relevant...
- ENFSI Verbal Scale
- A standardised table published by the European Network of Forensic Science Institutes that maps ranges of LR values to verbal expressions of...
- Float Process
- The Pilkington 1959 manufacturing method in which molten glass is poured onto a bath of molten tin, producing a ribbon with two...
- Glass-Transition Temperature (Tg)
- The temperature below which a glass behaves as a rigid solid rather than a viscous liquid; a fundamental thermal property that varies...
- 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...
- 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 variability in RI or elemental composition measured at different locations within a single reference pane; characterised as part of the comparison to estab...
- Glass: Types, Properties and ManufactureThe natural variability in RI or elemental composition measured at different locations within a single glass pane; must be characterised before a between-sourc...
- Likelihood Ratios and Verbal Scales in Geological EvidenceThe natural variation observed among multiple samples collected from a single source location. Used to estimate the probability of the observed difference betw...