The Supreme Court of Canada has told the countrys prosecutors that expert witnesses, including police officers, must not give opinions based on their experience.The ruling, in a major drug case, has wide implications for the prosecution of serious crimes, from high-value theft to murder, in which experts are called on to draw inferences or analyze evidence before the court. Such testimony has been linked to wrongful convictions, and the court was unanimous in trying to call a halt to opinion it feels has been disguised as fact.