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 Since the blog itself is no longer active, I have reposted the complete series of posts here (pulling them from archive.org).

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.

There was then a series of 7 responses which I’ll present in sequence (with minor spelling corrections relative to the original posts):

Response #1:

Absolute is not a term we can use with regard to physical phenomena. It should be apparent enough to anyone who practices one of the comparative forensic sciences why this is the case.
In my own field of friction skin analysis, I am well aware of a phenomenon I would call approximation. Approximation occurs when one image approximates another yet is produced by a different source. In fingerprint comparison, there are three levels of detail and approximation is possible at each level.
Level 1 is the pattern area which consists of Arches, Loops and Whorls. It is the fact that all fingerprint patterns fall into these three pattern groupings which made fingerprint classification possible before the advent of Automated Fingerprint Identification Systems. It is not uncommon to find approximation at this level especially between twins. In fingerprints, pattern is one of those characteristics which is genetically determined. We have seen in identical twins all ten fingers having approximating patterns. This is a huge reason why fingerprint identification does not occur using level 1. No one would ever conclude “absolute uniqueness” occurs at this level.
The work horse for determining uniqueness falls on level 2. Level 2 consists of the individual ridge characteristics which occur within ridges. If you looked closely at the individual ridges, you would see that they end or fork at different places within the pattern. They also have different lengths which allow them to be taken as an individual characteristic called minutia. In comparing two samples it is easily seen that there kind and location will either appear the same when contributed from the same source or different when compared to a sample from a different source. The problem is that approximation does occur at this level as well. The larger the minutia grouping the less likely one is to find approximation with other prints.
A combination of different kinds of minutia is called a minutia grouping. Groupings can be any number of minutia up to and including all the minutia one might find in a fingerprint. That is, analysts have found minutia groupings from different sources which approximate. The studies demanded by the NAS report will determine the frequency and examiner may expect to encounter level 2 approximation within a given population.
In effecting an identification, Level 2 is not taken alone or in isolation. It is used in conjunction with Level 1 and Level 3. Level 3 consists of ridge path, the course a ridge takes throughout a print, and ridge morphology which deals with the shape and width of the ridge and finally poroscopy, which deals with the arrangement of pores along a ridge.
The problem with latent prints is that there may be insufficient information in the print to determine Level 1, level 2 may have a small number of minutia and the print may have sufficient distortion to undermine any trust one might place on 3rd level detail. Distortion may be responsible for creating approximation in 3rd level detail in prints from different sources.
These are physical problems which preclude ever arriving at a conclusion of “absolute uniqueness.” This is just based upon what we can see. The fact that our view is limited to begin with, and the fact that there are millions and billions of fingerprints we have never seen of which all suffer from distortion and some of which may be affected to bring about approximation, precludes anyone from making a conclusion of “absolute uniqueness.”
“Absolute uniqueness” is a conclusion which can only be made if one has analyzed all friction skin in all humans and animals (past, present and future)being fully aware of the degree of distortion they are dealing with from one print to another and have determined a sample of friction skin to be excluded from all donors save one. This is of course, physically impossible. Therefore, to utter such a conclusion is to make an incredible boast or to speak in ignorance of the physiology of friction skin and the mechanics of print deposition. From what I have written here, one can easily see why such a statement cannot be responsibly made.

Response #2:

The question can also be framed as, “is uniqueness or individuality necessary for a jury to reach a conclusion?” That’s certainly not the case in other forensic disciplines.

Response #3:

Absolute uniqueness is not necessary. Look at the DNA model, they have established a probability of source. That is all fingerprints have to do. James Osterburg published a paper regarding this back about 1970. He did it without the computer. Now with AFIS, the probability of a “level 2 point” occurring as a particular location should be easy to calculate. That is all that would be necessary to meet the NAS and Court guidelines.
I know I have oversimplified the process, but hope someone will go with it.

Response #4:

Jay Siegel
I agree with Roy Marzioli’s assessment of the state of fingerprint science. I believe that, by extension, this applies currently to all forms of “pattern” evidence (firearms and tool marks, bite marks, handwriting, hair morphology, footwear, tire treads, etc.). Not only do I believe that a conclusion of absolute inclusion is not scientifically justified but that it is not necessary. I don’t believe that the concept of “individualization” has any place in science and is not provable in the real world. Why is it necessary to offer a nonsupportable, unprovable conclusion in court, where there is so much at stake and juries are so easily mislead? When a forensic scientists analyzes evidence that is brought to the lab by a criminal investigator, there is a reason why the focus is on that person. (This can set up a situation that invites bias on the part of the examiner, but that is another issue). The fact that there are many similarities between a latent print lifted at a crime scene and a print from a suspect, with no unexplainable differences, should be testimony enough. This would presumably be one piece of corroborating evidence in a net of evidence being offered by the prosecutor. There is no need to “guild the lily” by adding the conclusion that there is no other fingerprint in the world that could be the source for the print from the crime scene. Offering unsupportable conclusions in court reinforces the idea that forensic science really isn’t science; it is a tool of the prosecution.

Response #5:

Several issues muddy the water of discussing individualization, two of which I’ll mention briefly here.
The first is the acculturation of fingerprint examiners to the idea of individualization (sorry, two of my degrees are in physical anthropology and that’s how my mind works). The hardship is that individualization is now dogma, a belief system with true believers and proselytizers, and not science. “Uniqueness”, in the forensic sense, is not provable; this is why every other science uses statistics to address issues where the population is either so large or so intractable that sampling (and, hence, statistical analysis) is required. Much like the arguments about evolution and creationism, it is almost impossible for some fingerprint analysts to accept that individualization is not attainable in the ultimate sense (“to the exclusion of all possible humans”). We therefore, in many cases, are fighting the wrong fight. Arguing about which interpretation model to use for a latent ignores the fact that you may be arguing with someone who won’t be “converted.” Culture change is very hard and very difficult to manage.
The second muddying factor is that individualization, in its most common sense, is easily understood by anyone. I was listening to a speaker say, “Individualization does not exist and is not possible,” and I thought to myself, “Wait until his car gets stolen; it won’t be any of the thousands of Ford Taurus’ that were stolen–it was HIS.” The paradox of mass production is that, although we know that many things are analytically identical (although not numerically so, that is, if we numbered them, we could refer to “Number 3″ or “Number 16″), we know which are OURS. When the valet drops off my car, I know it’s mine and can prove it even though I haven’t memorized my VIN. The proximity of individualization to the average person’s awareness has, I feel and in part, kept fingerprints insulated from a more rigorous scientific treatment.

Response #6:

“Faith is a cop-out. If the only way you can accept an assertion is by faith, then you are conceding that it can’t be taken on its own merits.”-Dan Barker, “Losing Faith in Faith”, 1992.
Response #7:

The issue of ‘uniqueness’ is an enormous and unfortunate distraction. Uniqueness is neither sufficient for making absolute statements about the source of a mark, nor is it necessary for making probabilistic statements.
Any mark left by the same source is unique as well. As Wittgenstein stated: “Roughly speaking, to say of two things that they are identical is nonsense and to say of one thing that it is identical with itself is to say nothing at all” (Tractatus, 5.5303).
When considering the inference of identity of source, what should be discussed is within-source and between-source variability (see relevant literature).

Anyone who has perused my website will know that my answer to the original question is ‘no, uniqueness is not needed’ for forensic work — in any domain. I agree completely with Charles Berger that it is a distraction, at best. The better approach, alluded to in his final sentence, conforms to the logical approach to evidence evaluation being based on the concepts of within-source and between-source variation. That is all we have, and it is all we need.

Footnotes:
  1. Original citation: http://myblogs.informa.com/forensics/2009/09/04/expert-testimony-in-pattern-evidence-cases-is-absolute-uniqueness-necessary/
  2. Archived version: https://web.archive.org/web/20100825230036/http://myblogs.informa.com/forensics/2009/09/04/expert-testimony-in-pattern-evidence-cases-is-absolute-uniqueness-necessary/
  3. National Research Council Committee on Identifying the Needs of the Forensic Science Community, Strengthening Forensic Science in the United States: a Path Forward, National Academy Press, Washington, D. C, 2009 (available here)

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