Sequential Gate Design
Definition
The casework workflow structure in which analytical methods are applied in order of increasing cost and complexity, with an exclusion possible at each gate; ensures that expensive LA-ICP-MS is applied only to samples that pass all prior tests.
- Field
- Glass comparison casework
- Order
- Increasing cost and complexity
- Feature
- Exclusion possible at each gate
- Final gate example
- LA-ICP-MS reserved for samples passing prior tests
- Purpose
- Efficient resource allocation across a sample set
Common questions
What earlier, cheaper tests typically precede LA-ICP-MS in this workflow?+
Refractive index measurement and density determination are usually applied first, since they are fast and low-cost, and a sample that already differs from the known glass on RI or density can be excluded without ever needing elemental analysis.
Why does gate order matter beyond just saving money?+
Running the cheapest, most discriminating tests first means examiner time and instrument access, which are limited resources, get spent only on samples that remain genuinely indistinguishable after basic screening, keeping turnaround times reasonable across a full evidence set.
Does a sample that passes every gate prove a common source?+
No, passing all gates means the glass is indistinguishable on every property tested, which is consistent with, but does not prove, a common source; glass comparison results are reported as a level of association, not individualisation.
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,...
- 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...
- 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...
- 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...
- 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...
- 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,...
- Mahalanobis Distance
- A multivariate distance measure that expresses the separation between two observations in units of the population standard deviation, accounting for covariance; used...
- NIST SRM 612
- National Institute of Standards and Technology Standard Reference Material 612 (Trace Elements in Glass); the primary calibration material for LA-ICP-MS glass comparison,...
- Secondary Transfer
- The movement of a glass fragment from a primary surface (which contacted the broken glass) to a secondary surface (which contacted the...
- Within-Source Variation
- The natural variation observed among multiple samples collected from a single source location. Used to estimate the probability of the observed difference...