Dr Sarah Cumbers is chief executive of the Royal Statistical Society (RSS), the UK’s professional body for statisticians and other data professionals, and a charity advocating for the importance of statistics and data in decision making. In this guest blog, Sarah explores the role statistics play in the legal process.
Statistics can play a critical role in both civil and criminal cases, yet their significance in the courtroom is often overlooked and can be subject to misunderstandings. This is the type of thing you might expect me to say, as chief executive of the UK’s professional body for statisticians and data professionals. But I continue to be struck by just how pivotal a role they can play in legal proceedings: from the interpretation of forensic evidence to the assessment of risk of harm from a particular exposure.
One high-profile example that brought this issue into the spotlight recently, is the case of Lucy Letby, the convicted neonatal nurse. While this is not the place to discuss the specifics of the case, it did raise serious concerns among statisticians — particularly regarding the interpretation of the duty rota, which showed Letby on shift during the incidents in question. The controversy highlighted a broader issue: the need for better understanding and appreciation of statistics and probability within the legal system.
At the RSS, we’ve long recognised this need. Our Statistics and the Law Section was formally established in 2015. This group played a key role in challenging flawed statistical evidence in the case of Sally Clark, where an expert witness incorrectly calculated the probability of two sudden infant deaths occurring in one family and incorrectly interpreted the meaning of this probability. The RSS raised concerns with the Lord Chancellor at the time, and the convictions were eventually overturned on appeal. The case remains one of the UK’s most well-known miscarriages of justice.
Statistics in public inquiries
Statisticians have contributed to other major legal inquiries as well. Former RSS President Professor Sir David Spiegelhalter was instrumental in the Harold Shipman inquiry, arguing that statistical monitoring could have identified suspicious patterns much earlier—potentially saving many lives. We’re very pleased that David will be giving the closing keynote at the Bar Council’s annual conference this month, to discuss the role of statistical reasoning in the law and reflect on his experiences.
In 2022, our Statistics and the Law Section published a report on the unique challenges of investigating medical misconduct cases, 'Healthcare serial killer or coincidence?'. These cases often involve significant statistical uncertainty, especially when it’s unclear whether a crime has even occurred. Unlike typical murder cases, the absence of clear-cut evidence makes statistical reasoning even more critical.
In cases where statistics are relevant, having a statistician involved — either as an expert witness or consultant — is essential. However, recognising when statistics are relevant in the first place requires a basic level of statistical literacy as the statistical nature of evidence may not be immediately obvious.
A foundational understanding
We’re not suggesting that legal professionals become statisticians. But having a foundational understanding of statistics and probability can help lawyers interpret and present evidence more effectively. Such understanding will also allow lawyers to confidently challenge expert witnesses more effectively, being able to properly assess the reliability of their evidence as well as help them recognise possible flaws in the case of the opposition.
The RSS has already taken steps to support this. We’ve developed a series of guides for legal practitioners, covering topics from basic probability to evaluating DNA evidence. We also collaborated with the Inns of Court College of Advocacy to produce a guide specifically for advocates and with the Royal Society on their guide to statistics in legal proceedings.
Still, we’re aware that for statisticians, there is more work to be done. We’re continuing to engage with organisations across the legal system — including the Bar Council — to raise awareness of the importance of statistical literacy. We’re currently exploring what further guidance or training might look like but just bringing statisticians and legal professionals together to have these conversations has been a much-needed first step.
If you have thoughts or feedback on this topic, do get in touch. Please email us at [email protected]
Dr Sarah Cumbers is Chief Executive of the Royal Statistical Society. With a background in evidence-based policy and practice, Sarah was previously the director of evidence and insights at the global charity, Lloyd’s Register Foundation. Previously Sarah spent much of her career at the National Institute for Health and Care Excellence.