Area of Law – Evidence
An Article about Expert evidence in criminal cases
EVIDENCE – EXPERT IN CRIMINAL MATTER
The use of experts is often helpful in criminal cases. An expert can be used to show that the conclusion that Police experts are drawing is not right or is only one of the possible explanations for something happenning. There are many types of experts from DNA experts to motor vehicle accident reconstruction or forensic computer experts. We have used many types of experts successfully in cases. It is always worth checking expert assumptions with someone who is not part of the prosecution as sometimes they are completely wrong.
Most decent experts when confronted with their errors or a different hypothesis will admit that either they got it wrong or that the other hypothesis is true. If they do not all their evidence becomes damaged by the fact they are not being reasonable.
Using experts is like using private investigators in criminal cases. It is not done often but when it is done it can have a great benefit for a defendant.
Following is some case law about experts:
Principles of admissibility
The relevant principles of law governing the admissibility of expert evidence were as stated in Makita (Australia) Pty. Ltd. v. Sprowles,  52 N.S.W.L.R. 705 at 743  by Heydon, J.A. where his Honour held:
“In short, if evidence tendered as expert opinion evidence is to be admissible, it must be agreed or demonstrated that there is a field of ‘specialised knowledge’; there must be an identified aspect of that field in which the witness demonstrates that by reason of specified training, study or experience, the witness has become an expert; the opinion proffered must be ‘wholly or substantially based on the witness’s expert knowledge’; so far as the opinion is based on facts ‘observed’ by the expert, they must be identified and admissibly proved by the expert, and so far as the opinion is based on ‘assumed’ or ‘accepted’ facts, they must be identified and proved in some other way; it must be established that the facts on which the opinion is based form a proper foundation for it; and the opinion of an expert requires demonstration or examination of the scientific or other intellectual basis of the conclusions reached: that is, the expert’s evidence must explain how the field of ‘specialised knowledge’ in which the witness is expert by reason of ‘training, study or experience’, and on which the opinion is ‘wholly or substantially based’, applies to the facts assumed or observed so as to produce the opinion propounded. If all these matters are not made explicit, it is not possible to be sure whether the opinion is based wholly or substantially on the expert’s specialised knowledge. If the court cannot be sure of that, the evidence is strictly speaking not admissible, and, so far as it is admissible, of diminished weight. And an attempt to make the basis of the opinion explicit may reveal that it is not based on specialised expert knowledge, but, to use Gleeson CJ’s characterisation of the evidence in HG v. The Queen (1999) 197 CLR 414(at 428 ), on ‘a combination of speculation, inference, personal and second-hand views as to the credibility of the complainant, and a process of reasoning which went well beyond the field of expertise’.”
And at :
“The basal principle is that what an expert gives is an opinion based on facts. Because of that, the expert must either prove by admissible means the facts on which the opinion is based, or state explicitly the assumptions as to fact on which the opinion is based. If other admissible evidence establishes that the matters assumed are ‘sufficiently like’ the matters established ‘to render the opinion of the expert of any value’, even though they may not correspond ‘with complete precision’, the opinion will be admissible and material: see generally Paric v. John Holland Constructions Pty. Ltd.  2 N.S.W.L.R. 505 at 509-510; Paric v. John Holland (Constructions) Pty. Ltd. (at 846; 87).One of the reasons why the facts proved must correlate to some degree with those assumed is that the expert’s conclusion must have some rational relationship with the facts proved.”