Between-Source Variation
Definition
The variation observed among samples from different locations in the reference population. Used to estimate the probability of the observed similarity under the different-source hypothesis.
- Measures
- Variation among different-source samples
- Compared against
- Relevant reference population
- Used in
- Likelihood ratio, different-source hypothesis
- Field
- Forensic geology and trace evidence statistics
Common questions
How does between-source variation feed into a likelihood ratio?+
It estimates how probable the observed similarity between two samples would be if they actually came from different sources, forming the denominator against which the same-source probability is compared.
Why does the choice of reference population matter for this measure?+
A reference population that is too narrow or unrepresentative of realistic alternative sources can understate or overstate between-source variation, skewing the resulting likelihood ratio.
Related terms
- 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...
- Numerator Hypothesis
- The prosecution hypothesis under which the LR numerator is calculated. In a source comparison it is typically: the questioned sample and the...
- Tippett Plot
- A calibration diagnostic plot that shows the cumulative distributions of LR values calculated for known same-source pairs and known different-source pairs. Well-calibrated...
- 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...