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Computers – the enduring error: output = truth


Computers – the enduring error: output = truth


September 21


06:30 pm - 07:00 pm

Click to Register:

BCS, The Chartered Institute for IT

BCS Law Specialist Group


Paul Marshall

Stephen Mason

Flora Page

Professor Harold Thimbleby

Martyn Thomas


6.30 – Introduction by Law SG Chair, Sam De Silva

6.35 – The session exploring the unwarranted presumption in law that computers are reliable, the reality revealed by the Post Office Horizon scandal and warnings for algorithmic decision-making and AI.

6.55 – Q&A

7.05 – Event close


The Post Office Horizon scandal has been a legal catastrophe, both civil and criminal. It is predicated on the legal presumption that computers are reliable. In this session the speakers will consider why the presumption came into being, where it came from and why it is wrong.

The speakers will also consider the consequences of the presumption in practice: where the problems lie and why, especially in relation to legal aid and widespread ignorance of electronic evidence within the legal profession.

The session will also briefly consider ‘AI’ and ‘algorithmic decision making’. The question is whether it is or is not a panacea and whether it will cause even greater problems.

AI might not be a panacea and may be a Pandora’s jar. Finally, the speakers will consider some problems with AI – indeterminate and unpredictable output not capable of replication, and the ramifications of AI in legal proceedings: that is, untestability.


Paul Marshall, Barrister, Cornerstone Barristers, 2-3 Gray’s Inn Square, Gray’s Inn

Stephen Mason is editor, with Professor Daniel Seng, of Electronic Evidence and Electronic Signatures (5th edn, Institute of Advanced Legal Studies for the SAS Humanities Digital Library, School of Advanced Study, University of London, 2021)

Flora Page is a criminal barrister at 23ES Chambers and a PhD candidate at UCL. She prosecutes at the highest CPS grade for junior counsel, but her practice is 50% defence. She volunteered to assist the team led by Paul Marshall in the Post Office appeals. She has been in criminal practice since 1999, and has also worked at the Law Commission, the Financial Conduct Authority and the University of Law.

Professor Harold Thimbleby is an internationally-recognised computer scientist who has spent his career trying to improve the quality (primarily the safety and usability) of computer systems. He has been an expert witness in several NHS court cases and investigations involving unreliable computer systems that have led to patient harms and, in some cases, to death. His latest book, Fix IT How to see and solve the problems of digital healthcare is published by Oxford University Press, 2012.

Martyn Thomas is a software engineer specialising in safety-critical systems and cyber-security. He is Emeritus Professor of Software Engineering at Gresham College.


BCS Law Specialist Group

Our events are for adults aged 16 years and over.

For overseas delegates who wish to attend the event, please note that BCS does not issue invitation letters.



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