Despite the ubiquity of computers in everyday life, resolving a dispute regarding the misuse or malfunction of a system remains hard to do well. A recent example of this is the, now concluded, Post Office trial about the dispute between Post Office Limited and subpostmasters who operate some Post Office branches on their behalf.
Subpostmasters offer more than postal services, namely savings accounts, payment facilities, identity verification, professional accreditation, and lottery services. These services can involve large amounts of money, and subpostmasters were held liable for losses at their branch. The issue is that the accounting is done by the Horizon accounting system, a centralised system operated by Post Office Limited, and subpostmasters claim that their losses are not the result of errors or fraud on their part but rather a malfunction or malicious access to Horizon.
This case is interesting not only because of its scale (a settlement agreement worth close to £58 million was reached) but also because it highlights the difficulty in reasoning about issues related to computer systems in court. The case motivated us to write a short paper presented at the Security Protocols Workshop earlier this year – “Transparency Enhancing Technologies to Make Security Protocols Work for Humans”. This work focused on how the liability of a party could be determined when something goes wrong, i.e., whether a customer is a victim of a flaw in the service provider’s system or whether the customer has tried to defraud the service provider.
Applying Bayesian thinking to dispute resolution
An intuitive way of thinking about this problem is to apply Bayesian reasoning. Jaynes makes a good argument that any logically consistent form of reasoning will lead to taking this approach. Following this approach, we can consider the odd’s form of Bayes’ theorem expressed in the following way.
There is a good reason for considering the odd’s form of Bayes’ theorem over its standard form – it doesn’t just tell you if someone is likely to be liable, but whether they are more likely to be liable than not: a key consideration in civil litigation. If a party is liable, the probability that there is evidence is high so what matters is the probability that if the party is not liable there would be the same evidence. Useful evidence is, therefore, evidence that is unlikely to exist for a party that is not liable.