Posted on Wednesday, September 17, 2008 - 10:23 pm:

"Do you agree that fingerprints cannot be scientifically individualized?"

I would not use those words.

When I walk to the edge of the street holding my loving wife's hand, I look both ways for approaching traffic before we cross. I might see a car about two blocks away driving at what appears to be 20 to 30 miles per hour in our direction, and I might make the absolutely positive decision that we can both walk across the street without getting killed by that car while crossing. I make that determination based on my cumulative knowledge, training and ability and I have zero doubt that I am absolutely correct. In scientific terms, that absolute determination is illogical because there do exist rocket cars with jet engines mounted in them... and I cannot know what all such rocket propelled vehicles look like in all countries... and there is some minute chance that the vehicle I observed approaching at an apparently slow speed is in actuality some insane rocket car jockey in a vehicle that appears similar to a normal car, and that insane driver might fire up the rocket engines and mow us both down before we get two steps across the street. However, that all said, I am able to still understand and appreciate that even though scientifically there exists some very small chance (probability) that we will be mowed down by the approaching car, I have heard no information about the existence of such cars in our area, and I believe the chances of that happening are so remote that based on my training, experience and ability, I am willing to completely discount that slight possibility in my determination that we will not be killed by the approaching car. I am willing to stake my life (and my loving wife's) on a decision that I consciously make to ignore whatever slight scientific possibility there exists that I am wrong.

This explanation is something that judges, attorneys, and members of juries can understand. This is also a statement that holds up to scientific probability scrutiny. In science there are no absolutes. You cannot prove that there could never be some unknown rocket car that could instantly accel and kill you both from two blocks away. You can, however, consider that there is some remote possibility, but that possibility is so small you are willing to completely discount it in the absence of competing information such as news reports that a rocket car operated by a reckless driver has been seen in your area.

I would say that all friction ridge skin is unique, just as all trees and flowers are unique if you examine them in minute enough detail. However, the uniqueness of friction ridge skin structure does not necessarily carry over into latent print impressions because latent prints are by their very nature degraded and incomplete representations of a portion of the friction ridge features in skin. The amount that the uniqueness of friction ridge skin is carried over into an impression varies greatly depending on the quantity of friction ridge skin area, and also on clarity impacted by matrix, substrate, deposition, environment and other factors. For many latent prints, trained to competency examiners make a no-value determination that basically means that if the impressions were to be compared against millions of record prints, the examiner would accept that there is a possibility that similar friction ridge structures (at least structures so close that they cannot be eliminated/differentiated) may be found. This is true of such no-value impressions despite the fact that the individual impression is unique and unlike no other impression in the world (unless it is a duplicate photographic image or something of that nature) and despite the fact that only one individual’s unique friction ridge skin actually made the no-value latent print.

For latent prints that I (as a trained to competency Latent Print Examiner) consider of value for identification, I can make a decision and testify to a positive identification. When I say positive identification, what I mean is that based on my training, experience and ability (which is a whole lot more for latent prints than for rocket cars), I consider the possibility that there is some person in the next room, or in some other country, who has the same friction ridge detail in a configuration that could have made the latent print in question, to be such a remote possibility that I am willing to completely ignore it, to completely discount it and state that I am identifying that specific person.

The words "individualize to the exclusion of all persons on earth" are not scientifically accurate for most latent prints from the strictly scientific probability standpoint. However, Latent Print Examiners can make positive identifications wherein they explain what they mean (that they have an impression which is beyond that no-value level where they would accept the possibility that multiple similar friction ridge structures could exist)... and communicate the identification in a manner that withstands scientific probability scrutiny. This is what DNA Examiners do when they are asked how they can state that a certain person is the donor of a DNA sample.

There are situations where scientific probability supports true individualization, even for impressions that Latent Print Examiners would normally consider of no value. An example would be wet latent prints in blood on the interior wall of a locked room where the donor population is limited to only the ten persons in the room, and where the Level 1 ridge flow, and anatomical information combined with the three Level 2 details present, enable you to eliminate nine of the ten persons, but the tenth person has friction ridge skin that corresponds to the impression.

Part of what some critics have been saying is that Latent Print Examiners are not scientific because we do not understand probabilities, and that because we ignore probabilities we cannot be accurate. They are wrong. We do understand probabilities. We are determining that the probability is unacceptably high that more than one person's friction ridge skin could have made an impression when we determine that an impression with Level 2 detail is of no-value. I believe our problem is a matter of communication about probabilities. In the future, I believe there will exist tools supporting probabilities involving friction ridge unit relationships. Until then, there is a large body of evidence supporting the ability of trained-to-competency Latent Print Examiners to accurately make identifications based on existing guidelines and standards.

Sorry for the long-winded answer. I am sure there are better analogies than crossing the street, and more eloquent explanations that can be made by other Latent Print Examiners... but I hope this message gets across the gist of what I am trying to say.

--Ed