When the prosecution's cell site evidence proves… nothing?

Our expert examined the reasoning behind the prosecution's interpretation and showed that the survey data could not distinguish between the scene and the alibi location. Evidence that appears to distinguish between two locations -but cannot do so in reality - runs the risk of misleading the court. Hence, what the prosecution had treated as probative had no probative value on the question that mattered. The expert's oral evidence contributed significantly to the defendant's acquittal.

The danger is in nodding cell site evidence through. A map, a list of cells and a firm conclusion can look like proof of location, but the weakness often sits in the step between the data and the conclusion. The observation that a cell served at one location but not another was mistaken for a safe conclusion; it was not.

The courts have drawn this line before. In R v Calland, presentation of observations like the location of cellsites close to particular locations without giving opinion on their meaning was criticised. It was not a neutral presentation of call data. Defence teams are finally starting to see how fruitful that ruling can be.

Our Principal Expert, Dr Matt Tart, has written extensively on this problem. His peer-reviewed paper on opinion evidence in cell site analysis, commended by the Chartered Society of Forensic Sciences, sets out the legal and regulatory framework within which opinion must be given.  

It contrasts the task-driven approach common in this field with a structured approach to inference applied to the same data. His warning is that implication is often presented as fact, and that a good deal of cell site evidence contains unsound opinion that does not comply with the Forensic Science Regulator's codes, which expressly cover cell site analysis, or with the rules of evidence.

Three points defence teams might like to consider:

• Test the inference, not just the data. Sound data can still fail to support the conclusion drawn from it.
• Ask what the method can and cannot distinguish. Can it separate the scene from the alibi location? If not, what is it actually proving?
• Instruct an expert early. Prompt scrutiny can be a game-changer in terms of defence strategy

Read the full case study to see how the prosecution's interpretation unravelled.

‍