Is Absolute Uniqueness Necessary?

Many years ago I came across an interesting, if a bit limited, discussion in a blog post entitled “Expert testimony in pattern evidence cases – is absolute uniqueness necessary?”1,2That post is from Sept 4, 2009 just after the publication of National Academy of Science’s report “Strengthening Forensic Science in the United States: a Path Forward”.3 The question posed is still quite relevant today. I would say that most forensic practitioners today would answer the basic question regarding ‘absolute uniqueness’ with a negative response, but their reasons behind that answer applies will still vary.

For many people, ‘absolute uniqueness’ is a ‘forbidden’ concept mostly because of some policy they must follow, or because of a more personal recognition of some (often vague) issue relating to the ‘limits of science’. For other people the issue is a well-defined matter of science and logic, being dictated by the nature of information, what information can tell us about the world, and how information can and should be used to update belief about something. For the latter group (which is steadily increasing in size as awareness and understanding increases), the concept of ‘absolute uniqueness’ is neither required, nor beneficial in forensic work.

There has been a LOT of discussion about this in recent years, but I found the blog post interesting at the time (and I don’t think too much has changed since then). Since the blog itself is no longer active, I have reposted the complete series of posts here (pulling them from

The topic started with a post from the moderator:

Expert testimony in pattern evidence cases – is absolute uniqueness necessary?
What information is needed to form a conclusion about an identification? Do conclusions require statistical data, as in DNA cases, to offer an opinion? Is it possible to state that two items of evidence come from a sole source? What may an expert opine when no statistical data is readily available and only experience suggests a conclusion? The National Academy report raises some profound questions and some intriguing research possibilities. But in the interim, while we wait for academics to study the multitude of pattern evidence forensic scientists encounter in their day to day work, who may report cases and testify in court? Readers are invited to speak to these issues.

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Impinging on the Court’s authority

One of the strangest arguments I have heard raised in argument against the logical approach is that it means the expert’s testimony will end up impinging upon the authority of the Court. I have heard this a few times recently. It is particularly troubling that this has come from lawyers. Unfortunately, this has always happened in a venue where I could not actually discuss the matter with them.

As an objection to the logical approach, this is the strangest argument I have ever heard, without a doubt. In fact, proper application of the logical approach is one of the few ways to ensure that this will not happen.

To clarify, it is important to first understand the concept of “usurping the role of the Court” which means, in essence, to improperly influence the court’s procedures and decision-making, often by speaking inappropriately to or about the ultimate issue. Or, in other words, to impinge on the Court’s authority. To be sure, there is a legitimate concern that this might happen, particularly when the court is listening to an expert. As a result, it has been discussed literally for years. This is certainly not a new idea.

In fact, it can be found in various codes and directives regarding expert evidence. Indeed, Justice Sopinka noted this issue in the 1994 R. v. Mohan ruling when he stated, in part, “There is also a concern inherent in the application of this criterion that experts not be permitted to usurp the functions of the trier of fact.” 

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Propositions — key to the evaluation process

One of the key elements in the logical approach to evidence evaluation are the propositions used for the evaluation. They are, in a certain sense, the most important part of the whole process. At the same time, they are also one of the least understood.

Scales balancing Evidence

Today’s post explores the concept of propositions. I will attempt to describe what they are, how they are used, why we don’t change them once set and why they matter so much, among other things… all from the perspective of forensic document examination (though also applicable to other forensic disciplines).
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