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Principles and practice of CSI – introduction

Now we are going to focus a little more on the types of evidence recovered at crime scenes and some of the core principles of scene examination.
Although we hedged about defining forensic science, we can define its purpose as being “to provide objective information on which reliable, evidence-based decisions can be made.” Forensic science deals with what is known as real or physical evidence. That is evidence based on the discovery and examination of material sources, in contrast to testimonial or documentary evidence, which is someone’s verbal or written account of something. For example, the statements of Mr. Dougan and Mr. Ward that were taken by the police are testimonial evidence. But there is an expectation that real evidence will arise from the postmortem, from the examination of the car at the crime scene unit’s garage, and possibly from this search of the grounds next to where the car was found.
Even if we accept that real evidence is more reliable than testimonial evidence, this does not mean that it is absolute or infallible. However it has another valuable property, namely that it opens up a huge range of potential evidence that is unseen or unrecognised by the layperson, including police officers. In the words of Dr. Paul Kirk, who was head of the criminology and then the criminalistics programme at Berkeley University, “whatever the criminal leaves, even unconsciously, will serve as a silent witness against him.” The quote from Kirk in 1974 is a restatement of a concept enshrined in traditional crime scene investigation, the Locard exchange principle, usually summarised as, “every contact leaves a trace.”
The idea is that contact between people and objects or people and other people will always result in the transfer of traces, such as hairs, fibres, and DNA. And these will then provide evidence to link the accused to the scene and victim. Locard did not actually say this. And there are many issues with the concept, ranging from the epistemological concerns surrounding “every” to the practical issues of finding the transferred materials and how to interpret them. The South Australian case of Edward Charles Splatt provides some good illustrations of these issues. Mr. Splatt was convicted of the 1977 rape and murder of Mrs. Rosa Amelia Simper in Adelaide. All of the evidence linking him to the murder scene in Mrs.
Simper’s bedroom fell into the Locard category. It included very characteristic microscopic particles found on his clothing and at the scene. However the original source of the particles was a factory adjacent to Mrs. Simper’s home and where Mr. Splatt worked. There was nothing that specifically linked the particles at the scene to Mr. Splatt rather than to environmental contamination. But this was not mentioned in any reports or in the evidence led at the trial. Mr. Splatt had his conviction overturned in 1984 following a Royal Commission. The Splatt case illustrates that the Locard exchange principle is not a law of nature nor an ever present tool of the forensic scientist. But neither is it devoid of meaning.
If we return to the quotation from Kirk, we could take a more pragmatic view that real evidence can indeed function as the silent witness and allow us to view things in the context of the story that they can tell. To achieve that, we first of all need to identify and recover those material objects at the scene that will be the basis of the derived real evidence and do so in a way that is itself reliable. This requires that the actions taken at the scene are designed to control, preserve, record, and recover to allow us to reconstruct the events that created the crime scene, either by themselves or with the aid of the examination of the recovered materials.
We will explore, control, preserve, record, and recover in part two and reconstruct in part three.

You now have a tool, but what do you do with it and is it enough on its own?

The Introduction opens by defining the purpose of forensic science. It introduces us to the concept of “evidence” and in particular what some authorities define as “real evidence” (meaning ‘real’ in the sense of actually existing as a thing, rather than in the sense of genuine, not artificial). This is an important concept to grasp as courts – as many of us do in our daily business – tend to use witness narratives, or what people have to say, as the basis of providing and obtaining information.

The accompanying resources use eye-witness misidentifications to illustrate the pitfalls of testimonial evidence.

This doesn’t mean real evidence is infallible, and the video illustrate how an uncritical reliance on the Locard Exchange Principle can have devastating consequences. To be effective, the real evidence can only become Kirk’s “Silent Witness” if the incident scene is uncompromised. We therefore close by providing another tool – “CoPRRR” – which captures the 5 fundamental principles of incident investigation, each of which will be discussed in the next steps.

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Introduction to Forensic Science

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