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The “forensic” in “forensic science” – part B

The "forensic" in "forensic science" - Part B
We discussed identity a little in the introduction to DNA in week three. But it is a theme common to all five modules, including the case narratives. In week one, we looked at identifying the things that had been created or altered in the incident. In week two, we dealt with classic example of personal identity evidence, fingerprints. Week three introduced us to modern identity evidence in the form of DNA. And week four dealt with toolmark and impression evidence, which are essentially about the identity of objects. Finally, week five was all about drugs and their chemical identity. The final part of this discussion on forensic evidence, therefore, is about what we mean by identity and forensic science.
Dictionary definitions of identity encompass the concept of being who or what a person or thing is, identification as the means by which identity is established, and individual as a single member of a class or single human being as opposed to a group. In recent times, the power of DNA to come close to establishing a biological sample as coming from an individual as previously described has stimulated a debate about whether in fact we can identify someone or something with absolute certainty. Part of the problem has been of our own making in forensic science. The concept of identity individual from DNA analysis is based in the frequencies of DNA profiles in the population.
These are derived by measuring the observed frequencies of values for the various STRs from samples of different ethnic groups. When the overall frequency of a profile is calculated on the basis of this data, it can often turn out to be a figure that’s rarer than one in 7 billion. But 7 billion is the estimated world population. There are very good reasons for this apparent contradiction. But they don’t really address the fact that it raises questions in the minds of the lay person about the validity of the database. The problem is much more serious with the physical evidence examples for two reasons.
Firstly, there is no database with the frequency of Galton points and fingerprints and far less of the random marks used in firearms to mark footwear and tyre print impression evidence. And secondly, for decades, examiners globally have used wording like, in my opinion the marks match to the exclusion of all other possible marks.
There’s a major epistemological issue here in that there is not only insufficient detail to convert that opinion of individual to an assured identity of the thing as unique. But the idea that it is possible or far less reasonable to collect every example in the world is just silly. These are not trivial issues. Some courts are refusing to admit statistical data on DNA population frequencies because of a concern that it invites the jury to fall prey to the prosecutor’s fallacy, which in simple terms is making the mistake of taking the rarity of a DNA profile as a measure of the innocence of the accused. The DNA profile of the blood stain on the accused’s shirt is very rare.
It is the same as the blood of the victim. Therefore, it must be the victim’s. Therefore, he must be guilty. On the other hand, some authorities are holding up DNA as thee gold standard for evidence interpretation and are demanding that areas such as fingerprint comparisons establish the same quantity of databases. And indeed, the very reliability and scientific basis of fingerprint examination have been questioned. The issues can be put in perspective by returning to what we explored in discussing whether forensic science is a science and the ideas contained in the [? Asomoff ?] paper available in the resources. Any theory is only as good as the evidence available to test it at the time.
And as our knowledge increases, so will our ability to be confident in the continuing acceptance or need to modify a theory. The 20th century philosopher, Karl Popper, provided a useful tool for thinking about the meaning of identity and individual. He used the existence of black swans to show how ideas depend on the state of knowledge at the time. Before the European expedition of the New World, it as a matter of common knowledge that all swans were white. That changed when birds that were clearly swans but which had black plumage were found in Australia.
This illustrates a rational way of looking at what is involved in moving from non-exclusion to justifiable belief that the forensic science examinations in a case have established an association. The discovery of blacks swans lead to a refinement into the definition of what is meant by swan– the identity of the biological thing, or using the language of week two, its class characteristics. However, if the question is about which individual swan, then one or more properties in addition to the class characteristics must be invoked. Where the boundary between swan and which swan is set is situational. And there is no reason why it will not vary on a case by case basis. If the question is, is this a swan?
Then the class characteristics are sufficient to show identity and answer the question. If it is, is it this individual swan. Then no, the class characteristics are not sufficient and at best we have to fall back on our corroborative fail to exclude mantra. Phrases such as cannot exclude are sometimes misinterpreted to mean therefore it must be an identification on an association. There are various statistical tools to use in attempting to quantify the weight that may be attached to a known exclusion, including Bayes and random match probabilities. However, these are beyond the scope of this course. There are important non-mathematical issues that can help to give us some guidance as to the weight of a non-exclusion.
For example, the crime car in the case was a Mazda CX5, a relatively new model and the range of one of the less common makes of cars sold in the UK. If there had been no evidence from the tyre impressions at the jetty other than that they belong to the class Mazda CX5, and if Mr. Ward had denied every being there, then you don’t need a statistician to tell you that it is not very likely, but not impossible, that they came from someone else’s car. And the non-exclusion of Mr. Ward becomes more significant. However, the marks were at a jetty a place where boats are launched. The CX5 happens to be quite a suitable vehicle for towing a boat.
So what if the investigation turned up a boat owner with a CX5 who had been at the jetty on the morning of the incident? These simple examples illustrate how the hypothesis of what happened and the strength of possible associations can change as more and more information becomes available. You can easily imagine a scenario where Mr. Ward denied being at the lochside. The crime scene examiners found the tyre impressions. The prosecution used this as an important part of this case. And the defence produced Mr. Smith who described how he had driven to the jetty that morning in his CX5 to check if it was going to be a suitable day to take out his boat.
And of course it all became somewhat irrelevant when Mr. Ward changed his story to admit to being by the jetty. Perhaps the weight to be accorded to this evidence is the nebulous and unquantifiable one that it is evidence of the unreliability of Mr. Ward’s statements.

Now let’s look at identity, identification and individualisation in relation to our evidence types. We will explore what these terms mean in relation to identification of people and things.

There is quite a lot of material in this video and we would recommend that you print out the transcript and perhaps watch the video a number of times to get a better understanding of the main points.

The concepts of identification with ‘absolute’ certainty and whether or not we can do this are discussed in relation to DNA and physical evidence such as fingerprints, tool marks etc.

To help you we have added the resources we presented in a previous step ‘What is forensic science?’ again in the ‘downloads’ and ‘see also’ resource sections at the bottom of the page for your reference.

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Introduction to Forensic Science

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