T H E
T A I L
Monday, October 29, 2001
Welcome to the twelfth "Detail," a weekly e-mail newsletter that greets latent print examiners around the globe every Monday morning. The purpose of the Detail is to help keep you informed of the current state of affairs in the latent print community, to provide an avenue to circulate original fingerprint-related articles, and to announce important events as they happen in our field.
This week we finish out a series of Details regarding whether or not the principles and methodologies of friction ridge skin analysis constitute scientific practice. Last week, we explored the fact that ALL latent prints (and inked prints for that matter) are partial and distorted. The identification of friction ridge skin has been extensively tested, and continues to be tested, even on a case-by-case basis through verification. The suitability of a particular impression for identification purposes is a subjective decision based on the ability of that particular examiner, but if there is sufficient quality and quantity of unique detail present, then individualization can occur.
This week we are taking a look at the comparison of latent print examination to DNA analysis. Along the same lines, we are addressing whether the use of statistics is appropriate to model biological uniqueness.
There are many ways to address the comparison of the science of DNA analysis with the science of fingerprint examination. The way I like to look at this issue is to begin at the smallest level and work outwards. In DNA, there are only 4 possible chemical components present at a specific location on the DNA strand. A, T, C, and G are the first letters of the specific proteins that compose DNA molecules. Individuality comes about when specific segments of the DNA molecule contain these 4 proteins arranged in a certain order that is statistically correlated within a population. By looking at more and more segments, the chances of duplication become smaller and smaller until virtual certainty is reached.
The major difference with fingerprints is, of course, that there are more than 4 possibilities at any given point. With ridge detail, we are looking at infinite variation at each point, which a statistical model can approach, but never depict with complete accuracy. Biological uniqueness, in essence, is more variable in fingerprints than in the sections of DNA which are used in identification.
Another key concept is the impact of distortion. What we examine are impressions of the source, not the actual source. This is important because distortion is brought into play during the transfer of detail, and statistical models have to attempt to account for that distortion. This is no easy task. AFIS systems are continually improving the algorithms used to account for different types of distortion, but it is important to remember that models can approach accounting for every possible impact of distortion, but they can never be perfect.
How statistical models CAN assist the science of fingerprints is in demonstrating that the theory of biological uniqueness is a valid theory. This has been done by many researchers over the years. Further, they demonstrate, as the models become more and more accurate, that the chances of two different areas of even very small pieces of friction skin having the same configuration approaches impossibility. And this is just on level 2 detail alone!
Next week we will be addressing the issue of utilizing simultaneous impressions (one or neither of which "stands alone") in order to effect an identification. Steve Ostrowski has been researching several angles on this topic, and has recently presented his findings to change his department's procedures to allow for their use. Next week he presents the results of a survey he conducted, the arguments for and against, his opinions, and the final outcome of his situation (if resolved).
If you would like to comment on comparing DNA to Fingerprints or on the use of statistics to model individuality, visit the discussion forum and express your thoughts.
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Until next Monday morning, don't work too hard or too little.