Proposition Pair
Definition
The two competing hypotheses that frame an evaluative opinion: typically a prosecution proposition (Hp) and a defence proposition (Hd). A compliant report must state both propositions explicitly before expressing the likelihood ratio.
- Standard labels
- Hp (prosecution) and Hd (defence)
- Role
- Frames the likelihood ratio calculation
- Reporting requirement
- Both propositions stated explicitly before the LR
- Governing standard
- ENFSI and similar evaluative-reporting guidelines
Common questions
Why must both propositions be stated before the likelihood ratio is given?+
A likelihood ratio has no meaning on its own; it is always the ratio of the evidence's probability under one proposition against the other, so a report that omits the propositions leaves the number impossible to interpret correctly.
Who is responsible for setting the proposition pair, the scientist or the court?+
Guidelines generally hold that the propositions should reflect the legal issue in dispute and are properly agreed with, or at least informed by, the instructing party, while the scientist remains responsible for assessing the evidence against whichever pair is adopted.
Related terms
- Accreditation Body
- A national or regional organisation that assesses laboratories against ISO/IEC 17025 and grants accreditation. Examples include UKAS (United Kingdom Accreditation Service), NIST-associated...
- ENFSI Guideline for Evaluative Reporting
- A consensus guidance document from the European Network of Forensic Science Institutes that defines how member laboratories should express evaluative opinions. It...
- ILAC G19
- A guidance document from the International Laboratory Accreditation Cooperation on the use of uncertainty of measurement in forensic science reporting. It requires...
- ISO 17025
- International standard for the general requirements for the competence of testing and calibration laboratories. Covers management system requirements (document control, internal audits,...
- Verbal Scale
- The ENFSI translation of numerical likelihood ratios into courtroom language: very strong support (LR over 10,000), strong support (LR 1,000-10,000), moderate support...