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Stephen Mason sets out how the legal presumption, which exposed widespread misunderstanding about the nature of computer failures and caused serious widespread injustice, came into effect
In England and Wales, courts consider computers, as a matter of law, to have been working correctly unless there is evidence to the contrary. It follows that evidence produced by computers is treated as reliable unless other evidence suggests otherwise: this is a rebuttable presumption. In principle, there is a low threshold for rebutting the presumption that computer evidence is reliable. If such a challenge succeeds, the burden of proof lies with the party relying upon such a document to prove it, thus proving its source’s integrity and reliability. In a criminal trial, that burden is to the criminal standard.
The presumption was finally revealed as being false in the December 2019 judgment in the group litigation Bates v The Post Office Ltd (No 6: Horizon Issues) Rev 1 [2019] EWHC 3408 (QB), where Mr Justice Fraser (as he then was) concluded [at 968] that it was possible that software errors in the Horizon IT system could have caused apparent shortfalls in branch accounts, rather than these being due to theft or fraud.
The legal presumption, as applied in practice, has exposed widespread misunderstanding about the nature of computer failures. Errors in computer systems, specifically software defects (bugs), may not be readily apparent and can be difficult to identify. Bugs may cause a computer system to work differently from the intended behaviour, with unexpected – sometimes devastating – consequences.
The effects of bugs in the Post Office Horizon IT computer system, for instance, were not readily observable or identifiable to subpostmasters and others prosecuted by the Post Office. This means that those prosecuted could not effectively (or at all) challenge the reliability or integrity of the Horizon IT system. They had no means of providing evidence to the court capable of rebutting the presumption. In practice, rebutting the legal presumption presents insuperable and costly problems for defendants.
It is unknown how many other prosecutions will have been affected by the presumption. The fact that a computer has failed may well not be obvious. Professors Ladkin, Littlewood, Thimbleby and Thomas have written eloquently about this in ‘The Law Commission presumption concerning the dependability of computer evidence’, Digital Evidence and Electronic Signature Law Review 17 (2020) 1-14, and further articles have been written by Professor Ladkin, ‘Robustness of software’, Digital Evidence and Electronic Signature Law Review 17 (2020) 15-24 and James Christie, ‘The Post Office Horizon IT scandal and the presumption of the dependability of computer evidence’, Digital Evidence and Electronic Signature Law Review 17 (2020) 49-70. These articles are essential reading for all lawyers.
Readers will be aware that s 69 of the Police and Criminal Evidence Act 1984 (PACE 1984) required the prosecution to prove that a computer was operating properly at the relevant time before a document produced by such a computer could be admitted as evidence. However, as the volumes of computer evidence increased, this requirement became difficult and undesirable.
In 1997, the Law Commission published Evidence in Criminal Proceedings: Hearsay and Related Topics. In reviewing the problems faced by prosecutors, the Law Commission considered the law to be unsatisfactory and expressed the view that PACE 1984 s 69 served ‘no useful purpose’. It proposed, at 13.13, that s 69 should be repealed (and not replaced) with the effect that:
‘In the absence of evidence to the contrary, the courts will presume that mechanical instruments were in order at the material time.’
The Law Commission considered that the words ‘mechanical instruments’ extended to include computers. Section 69 of PACE 1984 was duly repealed.
The Law Commission admitted, at 13.18, that there was a practical problem in challenging the evidence:
‘The question is, what sort of evidence must the defence adduce, and how realistic is it to suppose that the defence will be able to adduce it without any knowledge of the working of the machine? …. It could therefore take very little for the presumption to be rebutted, if the party against whom the evidence was adduced could not be expected to produce more.’
The Law Commission also said, at 13.23:
‘… that the burden would be interpreted in such a way as to ensure that the presumption did not result in a conviction merely because the defence had failed to adduce evidence of malfunction which it was in no position to adduce. We believe, as did the vast majority of our respondents, that such a regime would work fairly.’
It cannot be said that the regime has worked fairly.
The presumption fails to make the crucial distinction between computer hardware and software. In fact, the great majority of failures of computer systems are attributable to failures of software – as indeed was the case for the Post Office Horizon IT system.
There have been two responses to the presumption. The one I took was to indicate that the presumption was factually incorrect, and I began my research in time for the 2010 edition of Electronic Evidence. This is now covered in chapter 5 in the practitioner text I edit with Professor Daniel Seng, Electronic Evidence and Electronic Signatures (5th edition, Institute of Advanced Legal Studies for the SAS Humanities Digital Library, School of Advanced Study, University of London, 2021).
The alternative is to question the rationale of the Law Commission in reaching the conclusions it did in 1997. In researching this, it transpired that those responsible in the Law Commission have a great deal to answer for recommending the repeal of s 69 of PACE 1984 and replacing it with the presumption that computers are reliable.
James Christie compared the Law Commission’s consultation paper, Evidence in Criminal Proceedings: Hearsay and Related Topics – A Consultation Paper (No 138, 1995) to the recommendation reached in the 1997 report. He has demonstrated that (p 62 of his recent article ‘The Law Commission and section 69 of the Police and Criminal Evidence Act 1984’, 20 Digital Evidence and Electronic Signature Law Review (2023) 62-95):
‘A study of the papers and articles quoted in the consultation paper and final report reveals that the Law Commission misunderstood, or misrepresented, the opinions of the main sources cited as being in favour of repeal. The Law Commission failed to address the strongest arguments against repeal without replacement, …. It ignored the advice of the experts they cited who all argued that the focus of courts should be on the reliability of computer evidence, …. The Law Commission’s comments and conclusions revealed that they had not understood the nature of computers and complex software systems as described in the sources upon which they relied.’
The Crown Prosecution Service, Director of Public Prosecutions and Post Office wanted s 69 to be repealed and the presumption to be introduced. The responses are published in 20 Digital Evidence and Electronic Signature Law Review (2023) Documents Supplement.
The question for those responsible in the Law Commission at the time is this: why did it misrepresent the comments of experts to introduce the presumption?
The presumption has been the cause of widespread injustice.
The finder of fact acts as a moral agent, and central to this is that the findings by a court must be justifiable and meet the demands of rationality and ethics. After all, the legal system is supposed to be about fairness and justice, isn’t it? And as Professor Moorhead writes, it’s also about the ethics of good chaps, isn’t it? (Lawyer Watch, 17 June 2023.)
In 2020 Mr Alex Chalk MP invited barrister Paul Marshall to make recommendations to the Ministry of Justice regarding the probity of computer evidence (the Marshall Report). A report was duly prepared and submitted. It was published in 2021: Paul Marshall, James Christie, Peter Bernard Ladkin, Bev Littlewood, Stephen Mason, Martin Newby, Jonathan Rogers, Harold Thimbleby, Martyn Thomas CBE, ‘Recommendations for the probity of computer evidence’, 18 Digital Evidence and Electronic Signature Law Review (2021) 18-26. The Report is understood to have been referred by the Under-Secretary of State to the Attorney General and the Chair of the Criminal Procedure Rule Committee for their consideration.
On 17 May 2022, James Cartlidge, Parliamentary Under Secretary of State (Ministry of Justice) stated that the government had ‘no plans to review the presumption, as it has wide application and is rebuttable if there is evidence to the contrary’. If the government maintains this position, then it is about time that the recommendations set out in the Marshall Report submitted to the Ministry of Justice should be put into effect.
Stephen Mason no longer practises, and is currently a Visiting Researcher at the Centre for Technology, Robotics, Artificial Intelligence and the Law, National University of Singapore (although not physically in Singapore).
In England and Wales, courts consider computers, as a matter of law, to have been working correctly unless there is evidence to the contrary. It follows that evidence produced by computers is treated as reliable unless other evidence suggests otherwise: this is a rebuttable presumption. In principle, there is a low threshold for rebutting the presumption that computer evidence is reliable. If such a challenge succeeds, the burden of proof lies with the party relying upon such a document to prove it, thus proving its source’s integrity and reliability. In a criminal trial, that burden is to the criminal standard.
The presumption was finally revealed as being false in the December 2019 judgment in the group litigation Bates v The Post Office Ltd (No 6: Horizon Issues) Rev 1 [2019] EWHC 3408 (QB), where Mr Justice Fraser (as he then was) concluded [at 968] that it was possible that software errors in the Horizon IT system could have caused apparent shortfalls in branch accounts, rather than these being due to theft or fraud.
The legal presumption, as applied in practice, has exposed widespread misunderstanding about the nature of computer failures. Errors in computer systems, specifically software defects (bugs), may not be readily apparent and can be difficult to identify. Bugs may cause a computer system to work differently from the intended behaviour, with unexpected – sometimes devastating – consequences.
The effects of bugs in the Post Office Horizon IT computer system, for instance, were not readily observable or identifiable to subpostmasters and others prosecuted by the Post Office. This means that those prosecuted could not effectively (or at all) challenge the reliability or integrity of the Horizon IT system. They had no means of providing evidence to the court capable of rebutting the presumption. In practice, rebutting the legal presumption presents insuperable and costly problems for defendants.
It is unknown how many other prosecutions will have been affected by the presumption. The fact that a computer has failed may well not be obvious. Professors Ladkin, Littlewood, Thimbleby and Thomas have written eloquently about this in ‘The Law Commission presumption concerning the dependability of computer evidence’, Digital Evidence and Electronic Signature Law Review 17 (2020) 1-14, and further articles have been written by Professor Ladkin, ‘Robustness of software’, Digital Evidence and Electronic Signature Law Review 17 (2020) 15-24 and James Christie, ‘The Post Office Horizon IT scandal and the presumption of the dependability of computer evidence’, Digital Evidence and Electronic Signature Law Review 17 (2020) 49-70. These articles are essential reading for all lawyers.
Readers will be aware that s 69 of the Police and Criminal Evidence Act 1984 (PACE 1984) required the prosecution to prove that a computer was operating properly at the relevant time before a document produced by such a computer could be admitted as evidence. However, as the volumes of computer evidence increased, this requirement became difficult and undesirable.
In 1997, the Law Commission published Evidence in Criminal Proceedings: Hearsay and Related Topics. In reviewing the problems faced by prosecutors, the Law Commission considered the law to be unsatisfactory and expressed the view that PACE 1984 s 69 served ‘no useful purpose’. It proposed, at 13.13, that s 69 should be repealed (and not replaced) with the effect that:
‘In the absence of evidence to the contrary, the courts will presume that mechanical instruments were in order at the material time.’
The Law Commission considered that the words ‘mechanical instruments’ extended to include computers. Section 69 of PACE 1984 was duly repealed.
The Law Commission admitted, at 13.18, that there was a practical problem in challenging the evidence:
‘The question is, what sort of evidence must the defence adduce, and how realistic is it to suppose that the defence will be able to adduce it without any knowledge of the working of the machine? …. It could therefore take very little for the presumption to be rebutted, if the party against whom the evidence was adduced could not be expected to produce more.’
The Law Commission also said, at 13.23:
‘… that the burden would be interpreted in such a way as to ensure that the presumption did not result in a conviction merely because the defence had failed to adduce evidence of malfunction which it was in no position to adduce. We believe, as did the vast majority of our respondents, that such a regime would work fairly.’
It cannot be said that the regime has worked fairly.
The presumption fails to make the crucial distinction between computer hardware and software. In fact, the great majority of failures of computer systems are attributable to failures of software – as indeed was the case for the Post Office Horizon IT system.
There have been two responses to the presumption. The one I took was to indicate that the presumption was factually incorrect, and I began my research in time for the 2010 edition of Electronic Evidence. This is now covered in chapter 5 in the practitioner text I edit with Professor Daniel Seng, Electronic Evidence and Electronic Signatures (5th edition, Institute of Advanced Legal Studies for the SAS Humanities Digital Library, School of Advanced Study, University of London, 2021).
The alternative is to question the rationale of the Law Commission in reaching the conclusions it did in 1997. In researching this, it transpired that those responsible in the Law Commission have a great deal to answer for recommending the repeal of s 69 of PACE 1984 and replacing it with the presumption that computers are reliable.
James Christie compared the Law Commission’s consultation paper, Evidence in Criminal Proceedings: Hearsay and Related Topics – A Consultation Paper (No 138, 1995) to the recommendation reached in the 1997 report. He has demonstrated that (p 62 of his recent article ‘The Law Commission and section 69 of the Police and Criminal Evidence Act 1984’, 20 Digital Evidence and Electronic Signature Law Review (2023) 62-95):
‘A study of the papers and articles quoted in the consultation paper and final report reveals that the Law Commission misunderstood, or misrepresented, the opinions of the main sources cited as being in favour of repeal. The Law Commission failed to address the strongest arguments against repeal without replacement, …. It ignored the advice of the experts they cited who all argued that the focus of courts should be on the reliability of computer evidence, …. The Law Commission’s comments and conclusions revealed that they had not understood the nature of computers and complex software systems as described in the sources upon which they relied.’
The Crown Prosecution Service, Director of Public Prosecutions and Post Office wanted s 69 to be repealed and the presumption to be introduced. The responses are published in 20 Digital Evidence and Electronic Signature Law Review (2023) Documents Supplement.
The question for those responsible in the Law Commission at the time is this: why did it misrepresent the comments of experts to introduce the presumption?
The presumption has been the cause of widespread injustice.
The finder of fact acts as a moral agent, and central to this is that the findings by a court must be justifiable and meet the demands of rationality and ethics. After all, the legal system is supposed to be about fairness and justice, isn’t it? And as Professor Moorhead writes, it’s also about the ethics of good chaps, isn’t it? (Lawyer Watch, 17 June 2023.)
In 2020 Mr Alex Chalk MP invited barrister Paul Marshall to make recommendations to the Ministry of Justice regarding the probity of computer evidence (the Marshall Report). A report was duly prepared and submitted. It was published in 2021: Paul Marshall, James Christie, Peter Bernard Ladkin, Bev Littlewood, Stephen Mason, Martin Newby, Jonathan Rogers, Harold Thimbleby, Martyn Thomas CBE, ‘Recommendations for the probity of computer evidence’, 18 Digital Evidence and Electronic Signature Law Review (2021) 18-26. The Report is understood to have been referred by the Under-Secretary of State to the Attorney General and the Chair of the Criminal Procedure Rule Committee for their consideration.
On 17 May 2022, James Cartlidge, Parliamentary Under Secretary of State (Ministry of Justice) stated that the government had ‘no plans to review the presumption, as it has wide application and is rebuttable if there is evidence to the contrary’. If the government maintains this position, then it is about time that the recommendations set out in the Marshall Report submitted to the Ministry of Justice should be put into effect.
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