Numerator Hypothesis
Definition
The prosecution hypothesis under which the LR numerator is calculated. In a source comparison it is typically: the questioned sample and the reference sample come from the same source location.
- Role
- Prosecution proposition in a likelihood ratio
- Position
- Numerator of the LR
- Paired with
- Denominator (defence) hypothesis
- Typical form
- Questioned and reference samples share a common source
Common questions
How does the numerator hypothesis differ from the denominator hypothesis?+
The numerator hypothesis is the proposition favouring the prosecution, typically that the two samples share a common origin, while the denominator hypothesis is the competing proposition, typically that they come from different, unrelated sources. The likelihood ratio compares how probable the evidence is under each.
Who decides how the numerator hypothesis is worded in a given case?+
The exact wording is usually set by the court or agreed between the parties and the expert, since it must reflect the actual factual dispute in the case rather than a generic template; a poorly framed hypothesis can make the likelihood ratio irrelevant to the question the court is actually asking.
Does a high likelihood ratio prove the numerator hypothesis is true?+
No. The likelihood ratio only expresses how much more probable the evidence is under one hypothesis than the other. It must be combined with the prior probability of each hypothesis, which is a matter for the court, not the expert, to reach a conclusion about guilt or source.
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...
- 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...
- 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...