Posterior Probability
Definition
The probability, returned by FORDISC, that the unknown skull belongs to the classified reference group, given that it must belong to one of the loaded groups. A value of 0.90 means 90 per cent of skulls in the reference sample most similar to the unknown belong to that group.
- Tool
- FORDISC craniometric classification software
- Meaning
- Probability the skull belongs to the classified reference group
- Condition
- Assumes the skull belongs to one of the loaded reference groups
- Example value
- 0.90 means 90 percent of similar reference skulls belong to that group
Common questions
Why can a high posterior probability still be misleading?+
The calculation is conditional on the unknown skull belonging to one of the groups loaded into the analysis, so if the true population of origin was never included as a reference group, FORDISC will still return a confident-looking probability for the closest available match, which can overstate certainty.
How should an analyst report this value in court?+
It should be presented alongside the typicality probability and a clear statement of which reference groups were used, since posterior probability alone, without that context, can be misread by a jury as an absolute statement of ancestry rather than a relative fit among the groups considered.
Related terms
- Anterior Nasal Spine (ANS)
- A morphoscopic trait: the bony projection at the inferior border of the nasal aperture, scored on a 0-3 scale from absent to...
- Discriminant Function Analysis
- A multivariate statistical technique that derives linear combinations of measurements to maximally separate two or more groups (male and female). The resulting...
- FORDISC
- Forensic Discriminant Computer program, developed by Ousley and Jantz at the University of Tennessee Knoxville. Performs discriminant-function analysis on user-supplied craniometric measurements...
- Forensic Data Bank (FDB)
- The modern documented skeletal sample within FORDISC, built from cases submitted by US medical examiners since the late 1980s. Provides reference data...
- Howells Craniometric Database
- A dataset of 82 standard craniometric measurements on over 2,500 adult skulls from more than 30 population groups worldwide, assembled by W.W....
- Malar Tubercle
- A small bony protuberance on the inferior surface of the malar (zygomatic) bone, scored as present or absent. Higher frequency in East...
- Morphoscopic Traits
- Non-metric cranial features assessed on ordinal or nominal scales (anterior nasal spine, malar tubercle, nasal bone shape, post-bregmatic depression, inferior nasal aperture...
- Population Affinity
- The preferred modern forensic-anthropological term for the relationship between an unidentified individual's skeletal morphology and a modern reference population. Replaces the older...
- Typicality Probability
- In FORDISC analysis, the probability that the unknown skull is a typical member of the best-matching reference group, based on Mahalanobis distance....