Highest Bayesian probability of Megrahi guilt 23 per cent

[Two highly important articles have recently been posted on the website Three Sides to Every Story. The first is headed Why the Lockerbie bomb was loaded at Heathrow and Megrahi was innocent. The first two paragraphs and the last paragraph of the lengthy piece read as follows:]

It is slightly shocking that Morag Kerr's book, which gives the first ever convincing, evidence-based reconstruction of the Lockerbie bombing, has not been reviewed in a major UK-wide newspaper since coming out in December.

She completely rebuts the case which was pressed by the Crown and accepted by the Camp Zeist court against the late Abdelbaset al-Megrahi, a Libyan agent, who served [8] years in prison in Scotland after conviction.  She also shows how the crime was really committed: not by Megrahi loading the suitcase with the bomb at Malta, to be transferred at Frankfurt onto the plane to Heathrow that was set to go on to New York City, before detonating over Scotland, but rather by persons unknown spiriting the suitcase onto the plane at Heathrow by placing it in a luggage shed ready to go directly on board Pan Am 103 to New York City. (...)

You will have to read the book and judge the forensic complexities for yourself.  For my part, I am convinced that Kerr is the first person to accurately reconstruct the Lockerbie bombing.  It was a crime perpetrated at Heathrow, and an innocent man suffered for it.  It is a textbook case of a miscarriage of justice, featuring leads missed by the police, unfeasible reconstructions of events and incompetent experts, as well as misconstrued, unreliable evidence both material and eye-witness.  The judges constructed a circumstantial case by irrationally explaining away key exculpatory evidence.  Kerr's book is not only a triumph of critical, evidence-based investigation, but also an instructive example of how a miscarriage of justice can occur.

[The second article is headed Bayesian probability analysis of the guilty verdict against Megrahi for the Lockerbie bombing. The first two paragraph read as follows:]

In my first post about the Lockerbie bombing, I discussed Morag Kerr's book reconstructing the commission of the Lockerbie bombing and demonstrating the innocence of the convicted man, Abdelbaset al-Megrahi.  In common with most humanistic reasoning, neither the verdict that condemned him nor Kerr's argument for his exoneration deployed any arithmetic of probability in analysing the evidence.  I think the widespread lack of arithmetical analysis of evidence is a serious weakness in fields like criminal law and history.

In this, I am following Richard Carrier in his book Proving History.  I am persuaded by him that we ought not just to use adjectives like "possible" and "probable" when we debate which theories best explain the evidence before us on a contentious historical or forensic question.  Additionally, we should use Bayes' Theorem: using numbers to express our opinions, and multiplying and dividing them according to Bayes' formula in order to calculate our reckoning of which theory explains the evidence the best.  The three main virtues of Bayes' Theorem are that it forces the analyst of evidence to specify clearly how good they think a theory is at explaining the evidence; it enables them to put all the evidence together in a mathematically sound way; and, above all, it forces them to look for evidence that supports their theory better than alternative theories, thus helping them to overcome the common failure to give alternatives due consideration.  Of course, different people can have different opinions about probabilities: the virtue of Bayes is that it brings out exactly what people agree and disagree about, and thus focuses their debate productively on crucial areas of disagreement.

[The author then subjects the evidence against Megrahi to Bayesian probability analysis and concludes that the highest probability of guilt that the judges should have arrived at was 23% and concludes:]

The judges failed to use Bayesian reasoning, which would have shown them that, far from a series of improbabilities adding up to a proof of Megrahi's guilt, they should have multiplied them out to a much greater sense of doubt.  They failed to appreciate that the crime was such an unlikely one on principle, that iron-clad evidence of Megrahi's guilt was required to overcome the prior improbability: extraordinary claims require extraordinary evidence.  A circumstantial case built on improbabilities does not cut it.

If the judges had applied correct probabilistic reasoning to the facts they did have about an unaccompanied bag from Warsaw, then this would have neutralised the evidence of an unaccompanied bag coming from Malta.

Of course, if we included the evidence explained yesterday, and considered how probable it was, on a hypothesis of Megrahi's guilt, that a mysterious suitcase answering to the description of the bomb-case would be seen by a baggage-handler at Heathrow before the feeder flight from Frankfurt had even arrived, then it would only be fair to divide the 23% we have come to here by maybe 10 times, if not more.  Include all the evidence, and the probability of guilt is minimal.

Moreover, include a more realistic expectation of the probability of getting the bomb into the baggage system at Malta, and the probability drops again to a minuscule number.

In sum, even without the new understanding born of Kerr's investigation, Megrahi should not have been found guilty. With it, his innocence is proven.

Thus the worst mass-murder in British history, the killing of 270 people, should be regarded as an unsolved crime.