T H E
D E T A I L
Monday, July 15, 2002
Good morning via the "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.
1. Whether the scientific theory/technique can and has been tested.
2. Whether the theory/technique has undergone peer review/publication.
3. What the known or potential rate of error is.
4. Whether the theory/technique has gained general acceptance in the scientific community.
Fingerprint Identification Theory
The concept of identification is based on the familiar concept that nature does not repeat itself. Every tangible object or person is different in innumerable ways. The closer something is examined the more differences one will find. This difference or detail is information about the subject’s unique features.
Fingerprint Identification is a comparison of friction skin or impressions of friction skin for purposes of evaluating the similarities and non-similarities of the permanent characteristics contained therein. Multiple impressions of the same area of friction skin will contain the same information within the spatial relationships of the characteristics themselves that have been reproduced. Fingerprint identification is based on this principle and can best be defined with the following premises.
- Friction skin ridge detail is unique and permanent, from birth till death’s
- The arrangement of this detail is also unique and permanent.
- Providing sufficient detail is present in the impressions, identification is possible.
The limitation of the identification principle is found in the lack of information available for comparison. Theoretically, it is not possible to have all the information on any tangible item. Yet, providing that sufficient detailed information is present identification is possible. Fingerprint impressions often yield much more information that is statistically necessary for identification. This information is not necessarily limited simply to the Galton characteristics of ending ridges, bifurcations and dots. Other types of characteristics may also be present and these can include general patterns, ridge flow, and general ridge characteristics such as ridge edge structure and pores. Without this information comparisons, for identification purposes, cannot be made.
As with most statistical studies, it is not feasible or practical to take a concept to its ultimate test in order to figure reasonable odds for specific components of an investigation. To fingerprint every person in the world (6 billion) and compare friction skin is certainly not practicable nor is it necessary.
A test by Lockheed Martin Corporation showed that fingerprints can be reliably and statistically matched to each other provided sufficient information is available. This was applicable to both rolled fingerprints and partial fingerprints representing crime scene latent fingerprints. 50,000 prints were compared to an additional 50,000 prints. Subsequently, scores were calculated from the information available from the comparisons. The test used both rolled impressions of the last joint of the finger and a second test utilized just a fraction of the original impression. The test showed that identical prints and partial prints utilizing only a fraction of the information illustrated a high differentiation between identical and not identical prints. The test also inadvertently discovered multiple duplicate fingerprints in the database. The fingerprints used in the test were taken from the Federal Bureau of Investigation’s files.
A more practical test of fingerprint identification’s accuracy is preformed on a regular basis in many fingerprint identification bureaus. This involves the identification of the fingerprints of police personnel from crime scenes and related crime scene evidence. Often, using the search capabilities of an Automated Fingerprint Identification Computer Systems and their databases, fingerprints recovered from crime scenes and related evidence are identified to documented personnel before any knowledge of which personnel were associated with the scene. When a computer database is utilized for search and comparison results, the candidate is often generated from a database that contains millions of fingerprints. Similar to part of the Lockheed Martin test, these searched fingerprints often represent only a small fragment of a fingerprint or palmar friction skin. This aspect is an informal blind test, and even though it may not be formally structured, it nonetheless demonstrates the effectiveness and validity of the fingerprint identification process.
A second type of everyday practical testing is derived from the fact that latent prints that are registered into an automated fingerprint identification computer database (A.F.I.S.) will often “hit” with a high candidate score, prints that have already been identified. With these cases, the fingerprints, unknown at the time, will be searched against the database with negative results. These prints can be kept in the database to be compared to the database again, or compared to new exemplar prints entered into the database. If the prints are subsequently identified and the search print in the database is not deleted, these prints will often be inadvertently matched to the new exemplar prints as they are entered and searched in the database. The exemplar search is a routine protocol to ensure that duplicate prints are not already in the database. A second aspect of searching exemplar (10-prints) against an unsolved registered latent crime scene print is that often at the time of the original search, a suspect does not have fingerprints on file in the database. Once prints are obtained from suspects and complaints, matches are often made against the original listed suspect. This type of search is a general undirected search of the database. With this type of search of the unsolved latent print database, the most information available may be the unknown fingerprint’s pattern, and in some cases the finger position on a hand. No personal information would be used in a search. This further illustrates the testable concept of fingerprint identification. Since the database may contain many millions of fingerprint files, the only reasonable way for these identifications to be made is due to the effective concept of fingerprint identification. Simple random hits would fail to produce so many computer matches, as a random hit would entail millions-to-one odds, depending on the size of the database.
The actual comparison process for fingerprint identification is known as the ACE-V process and has been outlined in detail by D. Ashbaugh in the publication: Quantitative-Qualitative Friction Ridge Analysis (Ashbaugh 1999). While not every agency and examiner follows the exact terminology of the ACE-V methodology, it has been shown in training classes that the underlying concepts of ACE-V are indeed universal.
Biometric companies including Automated Fingerprint Identification Systems (A.F.I.S.) also test on a regular basis to determine the effectiveness of their latest system developments. Any deviation from the expected and established concepts of fingerprint identification would be of considerable interest to fingerprint examiners. No research has noted a defect in the theory of fingerprint identification.
The accuracy of fingerprint identification has been demonstrated to be a product of the complete identification methodological process. This includes verification of the conclusion of an examiner. Blind proficiency tests are not a routine part of all fingerprint examiner training. While it is thought that blind testing can be a useful training tool, it has not been shown to affect accuracy and reliability issues.
Fingerprint Identification and peer review
The scientific field of forensic identification has a worldwide organization “The International Association for Identification” that produces a bi-monthly journal called “Journal of Forensic Identification.” This organization and its publication include research on fingerprint identification. The general forensic scientist organization, the “American Academy of Forensic Sciences,” also has a publication called “Journal of Forensic Sciences” that includes information and research on fingerprint identification. Research on fingerprint identification dates back over 115 years.
Numerous other publications exist including such books as:
- Contrast: An Investigator’s Basic Reference Guide To Fingerprint
Identification Concepts. (Coppock 2001)
- Quantitative-Qualitative Friction Ridge Analysis (Ashbaugh 1999)
- Advances in Fingerprint Technology (Lee, Gaensslen, 1991)
Peer review in general also includes the verification process, which is a standard part of the comparison methodology. The verification process can include personnel from local, state, federal and sometimes international levels. It is not uncommon for a fingerprint to be both analyzed and/or verified by another expert in a different agency.
Fingerprint Identification known or potential rate of error
In regards to “blind” identification of police personnel at crime scenes they were associated with, misidentifications of this type are exceedingly rare. In decades of fingerprint comparisons the average expert fingerprint examiner may only see a few false or misidentifications. Considering that over a career many examiners compare hundreds of thousands of fingerprints with thousands of fingerprint identifications a low error rate is evident. The identification process also includes a verification process that further reduces the possibility of a misidentification error making itself part of the permanent police record. Most clerical errors that may cause identification problems or discrepancies are sorted and corrected at the verification level. The calculation of error rates should consider the complete comparison process. When known error rates of fingerprint identifications are calculated from permanent law enforcement records, one finds a very low rate. This highlights the accuracy and reliability of fingerprint identification.
Statistical models of rolled fingerprint impressions likely to match another fingerprint impression not of the same source have been published. It must be noted that these models only use a specific level or type of available characteristic called a Galton characteristic. Even with this limitation it has been noted the odds of a match of non-identical prints would be a 1 followed by 97 zeros (1x1097). (Lockheed Martin) This astronomical number illustrates the fact why fingerprint identification can be effected with much less information, such as with a small section of friction skin as is commonly found in latent fingerprint impressions, where the odds were calculated at approximately 1x1027.
“It (fingerprint statistics) is a very complex issue, … and there have been only a few attempts to construct a suitable statistical model (Osterburg et al., 1977; Sclove, 1979, 1980; Stoney and Thornton, 1986; Hrechak and McHugh, 1990: Mardia et al, 1992).” “Suppose a suspect has been chosen on the basis of fingerprint evidence alone. The underlying population from which the suspect has to considered as being selected has been argued as being that of the whole world (Kingston, 1964). However, Stoney and Thornton (1986) argued that it is rarely that case that a suspect would be chosen purely on the basis of fingerprint evidence. Normally, there would have been a small group of suspects which would have been isolated from the world population on the basis of other evidence, though Kingston (1988) disagreed with this. The fingerprint evidence has then to be considered relative to this small group only.” Aitken, 1995
The supporting statistical models do vary according to the information available. If the information in a fingerprint impression falls below a relative threshold the supporting statistics become insufficient for an identification to be effected. This threshold is relative to the individual quality of the impression itself and can be considered as the information in which the impression contains. A standard procedure in fingerprint comparisons, for purposes of identification, is a requirement for an analysis of the available information contained in the fingerprint impression. Experience and knowledgeable fingerprint experts will understand the implications of the statistical variances on the possibility of accurate fingerprint identifications. Fingerprints that are not of identification quality levels are quite common and are known as smudges.
Fingerprint Identification acceptance in the scientific community
In the United States of America all 50 States have been utilizing fingerprint identification for the purposes of individualization for many decades. Canada, along with Great Britain, also use fingerprint identification. Many other countries also utilize fingerprint identification to some degree for the purpose of individualization. Fingerprint identification has been in general practice for about 100 years.
Some biometric companies also utilize various aspects of the science of fingerprint identification in the development and engineering of their products.
Lockheed Martin, Federal Bureau of Investigation (Fingerprint Test)
Aitken, C.G.G. (1995) Statistics and the Evaluation of Evidence for Forensic Scientists:
John Wiley & Sons Ltd., West Sussex, England