"Computer says guilty" - an introduction to the evidential presumption that computers are operating correctly

The first in a series of posts on the Post Office Horizon prosecutions scandal

The Post Office prosecutions scandal is the United Kingdom’s greatest mass miscarriage of justice of our times.

The scandal, however, is also difficult to write about.

Partly this is because many of the personal and systemic failures in the scandal are so maddening that any attempt at objective explanation and detached commentary can quickly become a rant.

And it is partly because the matter is so complex that very few will have mastery of all the legal and other documents and evidence. For example, the key 2019 judgment of Mr Justice Fraser - a judgment which also happens to be one of the greatest forensic exercises undertaken by any modern judge - is over a thousand paragraphs long, even without its appendices.

Nonetheless, there have been some outstanding accounts and analyses of this sorry situation. In particular, the journalist Nick Wallis has produced a book which should be read widely on the mess. There is also now a statutory inquiry which is seeking to get to the bottom of what happened, and why it happened, and how it should not happen again.

The focus of many of the accounts and much of the commentary has, rightly, been on the numerous personal and systemic failures - especially those of the Post Office management and their lawyers, and those of the software provider Fujitsu.

Those personal and systemic failures are central to what happened: none of the miscarriages of justice would have occurred without decisions by individuals (and groups of individuals) which could and should have been made differently.

And some of those decisions are such that the individuals involved should themselves be prosecuted.

But this post - and the posts which will follow this, as part of a series - is on another failure which was part of the mix.

This is the failure of the law itself and of the procedures of the courts.

And if anything, this failure of the law itself and of the procedures of the courts makes the individual decision-makers more culpable - for they knew (or should have known) how harsh the applicable law and procedure would be on the defendants, but the defendants would be prosecuted anyway.

Nothing in an account and explanation of the applicable law and procedure should be taken to limit the culpability of the Post Office management and their lawyers, and of those at the software provider Fujitsu.

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In this first post let us start with what lawyers called a “presumption”.

The classic statement of this presumption is as follows:

“In the absence of evidence to the contrary, the courts will presume that mechanical instruments were in order at the material time”.

Here “mechanical instruments” include computers.

So, in other words, computers are presumed to be operating correctly, unless there is evidence to the contrary.

As a “presumption” this does not mean that the court will take this view each and every time, regardless of circumstance.

It is instead a starting-point which can be rebutted.

It is what the court will take to be the state of affairs, unless it is satisfied by evidence that it is not the state of affairs.

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There is nothing inherently wrong about a court using presumptions: indeed, without presumptions, the courts could not properly operate.

Presumptions keep almost all legal cases manageable. For example, a contract will be presumed not to be a fraudulent instrument, unless it is shown to be a fake; or a defendant may be presumed not to be insane, unless shown to be insane; and so on.

Presumptions tell us what will be taken to be the state of affairs - and which party has the onus of showing whether that state of affairs is not correct.

The problems with any presumption are in what it presumes, and in what is needed to rebut it.

If the presumption is unrealistic in and of itself, or if rebuttal is unrealistic, then the presumption converts from being something that assists the course of justice to something that causes miscarriages of justice.

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The presumption that computers are presumed to be operating correctly, unless there is evidence to the contrary is what lawyers call “a presumption of evidence”.

This means that a court can be satisfied that a relevant fact can be established just by computer records, unless there is evidence that the computer is not working properly.

And so when the computer record shows, for instance, a financial shortfall by postmaster or postmistress, the court will accept that as evidence of an actual shortfall - unless the defendant can show that the computer was not operating correctly.

In short, when the computer record is the essence of a prosecution case: computer says guilty.

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This evidential presumption has not always been part of English law.

In 1984 a law was passed which pointed this presumption in the opposite direction.

Section 69 of the Police and Criminal Evidence Act 1984 provided:

This provision pointed the presumption in the other direction: it was for the prosecution to show that the computer was operating correctly, and not for the defendant to show that computer was not operating correctly.

This section 69 replaced the old common law position where, as stated above, in the absence of evidence to the contrary, the courts will presume that mechanical instruments were in order at the material time.

Had section 69 still been part of the law when the Post Office brought its prosecutions of post-masters and post-mistresses then the course of those cases may well have been different.

Section 69, however, was repealed in 1999:

The effect of this repeal was that the old common law presumption returned, meaning that is was again for the defendant to show that a computer was not operating correctly, rather than for the prosecutor to show that the computer was operating correctly.

How this repeal came about, and whether the reasons for that repeal were sound, will be the subject of the next post in this series.

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Sources

Considerable assistance for this series of posts has come from the following articles:

The Law Commission presumption concerning the dependability of computer evidence (2020)

Recommendations for the probity of computer evidence (2021)

The legal rule that computers are presumed to be operating correctly – unforeseen and unjust consequences (2022)