Tuesday 24 September 2019

Naked Statistical Evidence

Consider the hypothetical scenario:
All 100 prisoners in a prison participate in a riot, and 99 of them participate in attacking and killing a guard (the other returned to his cell briefly after the riot). With the guard dead, all 100 prisoners then escape. The next day one of the prisoners is captured and charged with participating in the murder of the guard. While admitting to participating in the riot the prisoner claims that he was the one who was not involved in attacking the guard. In the absence of any other evidence there is 99% probability the prisoner is guilty. Is this sufficient to convict?
Christian Dahlman
The latest episode of the evidence podcast "Excited Utterance" has an excellent interview with our colleague Christian Dahlman of Lund University about this kind of "naked statistical evidence", available on itunes, and also here:


Christian contrasts the above kind of naked statistical evidence with forensic evidence, such as a footprint found at a crime scene whose pattern 'matches' that of a shoe worn by the suspect. Whereas the causal link between the statistical evidence and guilt goes from the former to the latter, the  causal link between the forensic evidence and guilt goes from the latter to the former:

This difference is central to the recent paper about the 'opportunity prior' that we co-authored with Christian. The fact that the suspect was at the prison means that he had the 'opportunity' to participate in the killing and that the prior probability for guilt given the naked statistical evidence is 99%.

Christian talks about his latest paper, and at the end of the interview (24:50), he defends the Bayesian approach to legal evidence against attacks from some legal scholars (this is something we also did in our recent paper on countering the ‘probabilistic paradoxes in legal reasoning’ with Bayesian networks).


Sunday 8 September 2019

Book Review: Pat Wiltshire’s “Traces: The memoirs of a forensic scientist and criminal investigator”

Gripping, scientifically rigorous and moving memoir of the world’s leading forensic palynologist. 

The quote on the back cover of this book says: “Nature will invariably give up her secrets to those of us who know where to look”. Pat Wiltshire, a truly ‘one of a kind’ forensic ecologist, is probably the most qualified person in the world when it comes to knowing where to look.

This book is both a (popular) science book and a personal life story. The science is a thorough introduction to multiple aspects of ecology (and notably palynology – the study of pollen and spores from plants and fungi) as well as a detailed description of the processes of forensic investigation and analysis. The personal story fully reveals how Pat become the person she is, including her motivations, regrets, and loves. The science and the memoirs are interwoven throughout the book and what links much of the narrative are the accounts of Pat’s forensic investigations that provide fascinating insights into a number of different crimes (including murders and rapes) that Pat has helped shed light on. There are also eight pages of colour photographs of Pat at most stages of her life in the middle of the book.

See the full review (on ResearchGate):
Book Review: Pat Wiltshire’s “Traces: The memoirs of a forensic scientist and criminal investigator”

Full pfd also available here: https://www.eecs.qmul.ac.uk/~norman/papers/Traces_Review.pdf

Note: There are different UK and US versions (with different titles). The US version has different grammar and no photographs, but unlike the UK version, the audio version is narrated by Pat

UK: “Traces: The memoirs of a forensic scientist and criminal investigator” Bonnier Books UK, 2019

USA: "Nature of Life and Death", Putnam House  G P Putnam's Sons 2019