What is an expert witness?

M. J. Atha - Qualifications Prinicpal Consultant

In court, an expert witness can provide evidence on areas beyond the normal experience of a jury. He or she is under a duty to be impartial, and to base his expert opinion on verifiable fact. The standards of credibility required for expert defence witnesses are often much higher than for prosecution 'experts'. In drugs cases in the criminal courts, Police Officers normally provide 'expert' evidence as to the value of drugs and how much is compatible with personal use. They base this information on their 'experience', which can be as little as 6 months, or gained over several years.
 

Street values

When giving street values of drugs, some police officers give the highest possible value of the smallest possible unit, a basis described as similar to 'Strawberries at Wimbledon'. These values are ostensibly based on information from informants, defendants in custody and occasionally on 'controlled buys' by undercover officers. However, officers may not believe prices quoted by defendants which are lower than the accepted norm. In seizing 'proceeds of drug trafficking' the courts will presume that a defendant convicted of a 'trafficking offence' will have already received as profit the 'street' value of any drugs seized.
 

Plant yields

In cases of cannabis cultivation, people growing more than a handful of plants are likely to face a charge of intent to supply, and custodial sentences are commonly handed out for people 'growing their own'. Potential yields are commonly determined from sample plants, which are not always representative of the crop. Although forensic evidence in this area is improving in quality, yields are commonly expressed as including leaf, as well as bud.
 

Personal consumption or intent to supply

Police officers commonly regard any quantity representing more than 1-2 days supply, and sometimes less than this, as indicating an intent to supply. They commonly base their consumption figures on mere guesswork. Previous surveys in this series since 1984 have allowed a variety of defence experts in drugs cases to quote authoritative figures on consumption and drugs prices, and many wrongful convictions have been avoided, or acceptable pleas negotiated, as a result. Some prosecution witnesses have used previous survey results in commenting on consumption of drugs.