The proof of forensic evidence depends upon a number of other areas of evidence.
First, there has to be proof establishing that the evidence adduced in Court can be traced back to the place where whatever incident is at issue took place. Second, usually, there has to be an expert to interpret the meaning of the forensic evidence.
Thus, suppose a bullet casing is to be adduced in a murder trial – in order for the casing to be admitted the party seeking adduction must show (1) the casing was found at the murder site and (2) the casing is relevant to some issue at trial. In all likelihood, the proof the casing was found at the murder site can be established by a police witness who identifies the casing as being found at the crime scene. The meaning of the bullet case, however, will likely require expert evidence to interpret and explain the casing.
It is worth noting that there is, or can be, a significant cultural gap between scientists and lawyers that impacts on their understanding of evidence. The point is well put by Chayko and Gulliver:[1]
"...forensic science and the law differ in their approaches to the concept of “proof”. The scientist demands a higher degree of empirical evidence to reach a conclusion. Through endless trial and error and tested techniques involving multiple factors and controlled methodology, scientific “proof” in a laboratory is stricter, requiring indisputable substantiation. In contrast, the law has, as it is often said, no laboratories for testing theories. “Legal Proof” is more practical, focussing on probable legal effective causation."
While proof beyond reasonable doubt may approach scientific proof, the balance of probabilities standard is very much lower than such as to satisfy a scientist. The contrasting meanings of “proof” must be remembered whenever dealing with scientific experts. That said, forensic evidence can be critical and indeed, in the proper case, dispositive.[2]
[2] R. v Perlett [2006] OJ No. 3498
[1] G.M. Chayko, E.D. Guilliver, Forensic Evidence in Canada (2d ed) (CLB, Aurora, 1999) 5
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