Parties must be vigilant when selecting expert witnesses and cross-examining opposing experts.

Expert witnesses play an integral role in the litigation process assisting the court on technical matters. They are not advocates for a particular position and have a paramount duty to give impartial evidence. A recent High Court case shows how things can go wrong when the parties don't have all of the information (Rodi v Western Australia [2018] HCA 44).

One expert, two estimates

Rodi involved a criminal prosecution in connection with 925 grams of cannabis found in the defendant's possession. The prosecution said the defendant intended to sell the cannabis; the defendant said it was for his own personal use, and had been harvested from two live plants found at his property.

A key issue therefore was how much cannabis could have been produced by the two plants. The prosecution's expert testified that typically a plant would yield between 100 and 400 grams; therefore the 925 grams found at the defendant's property could not have come solely from the two plants.

The defendant was convicted and sentenced to 12 months' imprisonment.

Almost two years later the defendant learned that the prosecution's expert had previously given evidence that one plant could produce 300 to 600 grams of useable cannabis, which was consistent with the defendant's account of having harvested 925 grams from two plants.

If known to the defendant at his trial, the prior inconsistent statements would have enabled him to challenge the expertise of the witness and undermine the witness' credibility. He would have embraced the witness' previous evidence which was consistent with his innocence.

In quashing the defendant's conviction, the High Court unanimously held that there was a significant possibility the defendant would have been acquitted if the expert's earlier evidence had been before the jury. The Court found that the expert's earlier contradictory opinion undermined the value of his evidence and called into question his expertise.

Doing your due diligence on your expert witnesses

There were no winners in the case. The prosecution's case was undermined by the prior inconsistent evidence and arguably should never have proceeded. The defendant served a term of imprisonment based on evidence which was fundamentally compromised.

Rodi shows the importance of thoroughly understanding and checking a witness' expertise. This would include asking about (and researching, as appropriate) whether a witness has previously expressed a different view. It is not enough to trust that a witness will bring this to the parties' or the court's attention.

The need to carefully evaluate expertise, and test admissibility and weight is equally relevant in criminal and civil cases although the consequences may vary. The case is a reminder of the need for lawyers to do their homework when selecting or cross-examining an expert.

There has been a growth of businesses which collect and analyse information about expert witnesses. These businesses apply the type of data analytics usually associated with mainstream commercial enterprises. Such a tool would likely have uncovered the prior inconsistent statements in the Rodi case.

As services of this kind become increasingly common, lawyers may have a duty to use such services or at least advise clients of the option to do so.

The importance of careful due diligence cannot be overstated given the potential consequences as demonstrated in the Rodi case.