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The Examination of Fingerprints
by ALAN MCROBERTS One area of Forensic Identification is the examination of friction ridge detail to identify the origin, or person who left the print. This friction ridge detail includes marks of the surfaces of fingers, and palms of hands, and soles of feet discovered as evidence at a crime scene or from items associated with a criminal investigation. These friction ridge marks, unintentionally left behind by the owner, are frequently of poor quality when evaluated against the intentional exemplar prints with which most people are familiar. The examination of these unintentional marks requires a thorough process to insure proper identification of their origin. The methodology as described by Ashbaugh is a three--phase process. The examiner will initially “analyze” the evidence, and then visually “compare” the evidence and exemplar for similarities or dissimilarities. In the final phase, the examiner will “evaluate” the comparison and render a conclusion (28). The first step in the analysis phase is to inspect the evidence to ascertain that the detail being examined is, in fact, a representation (whether photograph, latent lift, or other recording) of friction ridge detail (more technically known as volar friction skin detail). Many other sources can also produce similar appearing detail. “There are numerous parallelisms in nature of dermatoglyphic configurations and of minutiae of individual ridges” (Cummins, 34). While Cummins was addressing these parallelisms for a different purpose, the fact remains that, in nature, and in manufactured items, there are common sources of marks (like gloves) which can leave a mark that, without thorough examination, can appear as friction ridge detail. The second step within the analysis phase is to recognize the various factors that may have affected the appearance of the detail. The substrate, on which the print was deposited, and subsequently developed, can provide the explanation for many artifacts and distortions that may exist within the print area. The constituents of the print itself may be of significant importance. A print of natural volar skin exudes will appear differently from a print deposited with a thick medium such as blood. The pressure which was exerted when the print was placed is a major factor in how the print will appear. A print placed with excessive pressure may cause the constituents of the print to squeeze into the furrows between the ridges and cause what is known as a reversal print. Unless recognized as a reversal print, the print can be easily overlooked, and a missed identification would occur. The final step of the analysis phase is to decide what is ridge detail. Taking into consideration the factors identified in the second step, the examiner must interpret the visual information and differentiate between what is friction ridge detail in the print under examination, and what is not. When overlays of two prints have occurred, determination of which print a particular element of ridge structure belongs to is critical to the success of the examination. Recognizing what does not belong to the print in question is equally important. If a particular feature does not belong to the print being examined, that print is doomed to never be identified. The second phase of the examination process is to compare and detect differences or resemblances between the evidence print and the known exemplars of any person thought to have made the print. Initially, the examiner will begin the comparison by thorough use of class characteristics. “Class characteristics of friction ridge prints are those characteristics that can be possessed by more than one print” (Cowger, 129). When present, class characteristics will allow the examiner to quickly eliminate areas on the exemplar(s) with dissimilar class characteristics as not being from the same source. If an evidence print depicts one of the three general pattern types (arches, loops and whorls), any exemplar or individual print not having that pattern is quickly removed from future comparisons against that print. While not all prints will have pattern types, most will have some general ridge flow that also can be used as a class characteristic. This ridge flow is also useful in determining from what skin area the print originated. Each area of volar friction skin on the fingers and palms of hands, and soles of the feet have customary ridge flows that are characteristic of their location. After determining the proper skin area, the pattern, or ridge flow, can be further utilized to orient the evidence in alignment with the exemplar. After the class characteristics have been used to narrow the search to the correct skin area, and the evidence print has been oriented, the examiner will select a focal, or reference point, with which to begin the comparison. This starting point will frequently be at the core, a triradius, or other prominent feature. A comparison of the individual characteristics, or “characteristics that are unique” (Tuthill, 41) is then initiated. First, the examiner selects an individual characteristic within the evidence print, and then locates the same characteristic on the exemplar. Returning to the evidence print, the examiner will then follow that (or an adjoining) ridge until a deviation of path or another unique feature is located. These features may be a bifurcation of the ridge, an ending ridge, an adjacent dot (a short ridge having a length no longer than its width), a notable feature along the edge of the ridge, or distinctive pores atop the ridge. The comparison phase will continue until the entire evidence print has been compared, or until sufficient area has been examined to support a definitive conclusion during the subsequent evaluation phase. The final phase of the examination is the evaluation of the visual information obtained. While this phase is completed after the comparison phase, various evaluations begin within the initial analysis phase, and continue throughout the examination. Class characteristics will be evaluated for rarity of pattern type and ridge flow. Individual characteristics will be evaluated for clarity. These evaluations will dictate the weight given to both class and individual characteristics used in the comparison. The evaluation of a completed comparison finalizes with one of three conclusions. The prevailing community standard is reflected in the guidelines put forth by the Federally sponsored T.W.G.F.A.S.T. committee, which declares there are only three acceptable conclusions: To formulate the conclusion, a sufficient quantity of detail must be obtained in the comparison phase. The amount of detail necessary will vary from print to print, depending on the clarity, uniqueness of characteristics, and ridge arrangement. A non--identification will require only a single class characteristic or unique feature in the evidence, which can be determined not to exist with the donor of the exemplar. Conversely, the detail to establish an identification will normally be more substantial. The examiner must be individually convinced, to a moral certainty, that the prints compared to each other were made by the same person. This conclusion is one of the most highly respected and least challenged forensic opinions. It can be, and should be, a professional tragedy for a qualified examiner to erroneously declare an identification, when one does not exist. The concluding portion of the evaluation phase is not truly an evaluation by the examiner, but the verification or evaluation of the findings by a second examiner. As basic scientific principles require repeatability of tests, the field of Forensic Identification requires that the repeatability be demonstrated when the results of a comparison conclude with an identification. “All identifications must be verified by a qualified latent print examiner” (U.S. Gov., T.W.G.F.A.S.T., 430). The identification of friction ridge prints requires a thorough examination process to insure proper determination of origin. The examination methodology is described as a three--phase process. The examiner will initially analyze the evidence and then proceed to compare the evidence against the exemplar(s). In the final phase, the examiner will evaluate the comparison and render a conclusion. Simply stated, the examiner will analyze, compare and evaluate the evidence.
References: Ashbaugh, David R. “Ridgeology--Modern Evaluative Friction Ridge Identification.” JFI 41, (1991): 16--64. Cowger, James F. Friction Ridge Skin -- Comparison and Identification of Fingerprints. New York: Elsevier, 1983. Cummins, Harold and Charles Midlo. Finger Prints, Palms and Soles. 1943. New York: Dover, 1961. Tuthill, Harold. Individualization: Principles and Procedures in Criminalistics. Salem, Oregon: Lightning Powder, 1994. U.S. Department of Justice, Technical Working Group on Friction Ridge Analysis, Study and Technology. Proposed TWGFAST Guidelines. JFI 47, (1997): 425--437.
This original article was printed in “THE PRINT” |