Denominator Hypothesis
Definition
The defence hypothesis under which the LR denominator is calculated. Typically: the questioned sample comes from some other location within the relevant reference population.
- Role
- Alternative (defence) hypothesis in a likelihood ratio
- Context
- Geological trace evidence evaluation
- Typical form
- Sample originated from another location
Common questions
How does the denominator hypothesis affect the size of a likelihood ratio?+
A broader or more probable denominator hypothesis, such as a common soil type found across a wide area, lowers the resulting likelihood ratio, while a narrow, improbable alternative raises it, so the choice of hypothesis strongly shapes the reported strength of evidence.
Who is responsible for framing the denominator hypothesis in a case?+
The expert typically frames it based on case circumstances and instructions from the parties, and it should represent a genuinely realistic alternative source rather than an arbitrary or unlikely scenario, since an unrealistic denominator can distort the likelihood ratio.
Related terms
- 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...
- 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...