Experts in the dock’, screamed one broadsheet headline in January 2004 after the Court of Appeal quashed Angela Cannings’ conviction for murdering two of her children. The Court of Appeal said that in cases such as Cannings’ it will often be unsafe to proceed to trial if the outcome would depend on a serious disagreement between distinguished and reputable experts.

The court made that observation because Cannings’ case was the third of its type in 2003. But to see the Sally Clark, Trupti Patel and Cannings cases as identical is too simple. The verdict in Patel decided no point of principle: on the evidence the jury was not sure of guilt. Save superficially, the quashing of Sally Clark’s conviction and the Cannings appeal were dissimilar and raised different issues.

In Clark, the Court of Appeal was presented with evidence of apparent misconduct and serious non-disclosure by an expert witness called by the Crown, Dr Williams, which came to light after conviction. That would have been sufficient for the conviction to be quashed. In addition, expert evidence describing statistical prob-abilities was also severely criticised. That evidence was given by Professor Sir Roy Meadow, described by the Court of Appeal as an expert witness of great distinction, if not pre-eminence, in this field.

Meadow’s evidence in the Cannings case did not extend to the flawed statistical evidence presented to the jury during the trial of Clark.

In reality, the Crown’s case against Cannings depended on specialist evidence about the conclusions to be drawn from a history of three infant deaths and further ‘acute’ or ‘apparent life threatening events’ in the same family.

The release of Cannings put local authority care proceedings and adoptions under scrutiny to an extent that prompted The Guardian to say: “[I]t looked as if social services departments throughout the land were heading for meltdown.” The Attorney-General Lord Goldsmith ordered a review of 258 cases where a parent had been convicted within the last 10 years of killing a child under two. A day later, Solicitor-General Harriet Harman told MPs about the implications for child protection. If it was unsafe to convict parents under criminal law on the basis of misguided expert evidence, it must also be wrong to take children away from parents under civil law on the same grounds. Harman had in mind cases where suspected mothers were not prosecuted for harming their children, but had them removed in care proceedings brought by a local authority. The first woman to go to the Court of Appeal as a result of the Cannings judgment lost her child. A judge ruled on 25 March that the two-year-old girl should be freed for adoption.

The reality is that courts are often dependent for their decisions on the quality of the medical evidence they receive and the integrity of those who provide it. It is worthwhile for practitioners and expert witnesses to remind themselves of the nature, purpose and limitations of expert evidence.

Expert evidence is an exception to the common law rule that a witness gives evidence only as to fact. Where a person is called as a witness in any civil proceedings, their opinion on any relevant matter on which they are qualified to give expert evidence shall be admissible in evidence. An expert assists the court by providing an opinion about a question that is not within the skill and experience of the court. They are under a general duty to provide that independent assistance by way of objective, unbiased opinion in relation to matters within their expertise. Experts should make it clear when a particular question or issue falls outside their expertise. In public law Children Act cases, the expert has a particular duty, under the act, “to confine an opinion to matters material to the issues between the parties and in relation only to questions that are within the expert’s expertise (skill and experience). If a question is put which falls outside that expertise, the expert must say so.”

The expert witnesses who are most likely to assist the court and are least likely to be criticised will perhaps be those who study most carefully and comply with their duties to the court.

David Boyd, barrister, Coram Chambers Professor