Friction Skin Identification
A Scientific Approach

(The following paper is presented as a rebuttal to recent suggestions in Identification literature and training which tend or attempt to promote a set minimum number of characteristics.)

By WILLIAM F. LEO C.L.P.E.
Los Angeles County Sheriff's Department
Scientific Services Bureau --— Identification Section

Friction skin identification has evolved from merely an established set number of characteristics identifying only fingerprints to identifying any area of friction skin through an advanced philosophy of identification.  This philosophy is based on the scientific research into the formation of friction skin, improved techniques of latent print development, and the refinement of our ability to perform comparative analysis of more detailed ridge structure.  

As very well stated by Ashbaugh in the text Ridgeology, there are three stages to the identification process.  They are: analysis, comparison, and evaluation.  The latent print is analyzed to determine the area of friction skin it may have come from, the detail present, and the clarity of that detail.  The latent print is then compared with the known exemplar.  At this time similarities or dissimilarities between the latent and exemplar are evaluated to establish an identification or elimination.

The most important element in friction skin identification is the experience of the examiner. The latent print examiner must be thoroughly trained in how friction skin is formed and why all areas of friction skin are unique and permanent.  By having a thorough knowledge of the scientific basis of friction skin identification, latent print examiners are better able to explain and defend their conclusions with confidence on the witness stand.  To obtain quality experience, this training must be followed by extensive supervised comparison experience.

Verification also plays a major role in the scientific process. Error in identification is unacceptable.  In any science, the ability to duplicate findings is one accepted way to validate initial findings.  The latent print examiner who is given the task of verification must be an experienced examiner and must understand the importance of the verification process.

Just as every area of friction skin is unique, so is every print comparison.  During the examination process the latent print examiner should avoid any artificial parameters which do not have scientific basis, such as an examiner having a predetermined number of characteristics needed to establish an identification.  Once again, every area of friction skin is unique and so is every comparison.  

The vast majority of observations made during the identification process do not have numeric value.  Placement of ridge characteristics, their relationships, clarity, uniqueness,   ridge flow, overall pattern type and groupings of characteristics all are analyzed and compared during the identification process.  Areas void of ridge characteristics are observed and the ridges between the major characteristics are counted and observed.  All of this information is evaluated, but it is not given an accumulative numeric value, yet all is equally important.  The  flaw in the use of any numeric standard for friction ridge identification is its inability to account for all the observations made and the examiner's ability to evaluate this information.

The concept that minimum number standards prevent errors in identification is also flawed.  The competent examiner is able to identify with minimal information, while the incompetent or inexperienced examiner has misidentified with an abundance of characteristics.               

Listed below are references made by respected members of the identification community as to why we do not or should not use any predetermined number standard as a basis for friction skin identification:

“The demand for twelve similar details is the result of the opinion of bygone days founded on the belief of scientists such as Galton, Remus, Balthazard, and others.  All recent scientists working in the field of dactyloscopy, as for instance, Locard, DeRichter, and others, share the opinion that the number of characteristic points which can be noted at the edge of an enlargement is a matter of little importance.”1

“There is no valid scientific basis for requiring  a minimum number of ridge characteristics which must be present in two fingerprints in order to establish positive identification.”2

“The clearer the friction ridges available for comparison, the greater the opportunity there may be to compare and evaluate minute detail.  The more minute detail found in agreement, the greater the individualizing power of that area of friction ridge.  The more obscure and undetailed the ridge structures the less individualizing power it has.  It is therefore possible for an opinion to be formed on differing areas of friction ridges due to the quality of the friction ridge structures and the quantity of the ridge details present.”
“Different levels of knowledge and experience coupled with available quality and quantity of ridge detail dictates that a preset number or size of ridge detail cannot be established as a basis for identification.  Examiners of equal experience and training should arrive at an identical conclusion when comparing the same area of friction ridges.”3

“With experience,  the examiner will utilize the qualities of the entire image, including ridge contour and pores, to augment or diminish the assessed quantitative aspects.  A latent with fewer characteristics of quality may be identified with confidence in preference to a latent with a higher number of common, but ambiguous points.  Qualitative and quantitative analyses may shift in relative importance according to the latent evaluated, but must serve the examiner in harmony.  Artificial standards dismiss the qualitative factor by establishing inflated quantitative requirements to compensate for a worst case scenario of characteristic quality.  Whether artificial standards are imposed from outside or within the examiner, the primary rationale offered is the prevention of identification error.  In the guise of quality control, standards based solely upon the quantity of characteristics reduce an examiner to a technician more adept at ciphering than exercising scientific judgement.

“Despite the multiplicity of standards and methods of calculation, there exists no universally acceptable number of ridge characteristics which can be required in every identification in every case.”5

“No valid basis exist at this time for requiring that a pre--determined minimum number of friction ridge characteristics must be present in two impressions in order to establish a positive identification.”6

“It has been suggested by some laypeople that latent print identification may not be scientific because there is no minimum standard regarding the total number of ridge characteristics required for a positive identification.  This belief reflects a simplistic view that the identification process is merely the totaling of clearly defined ridge characteristics to obtain an arbitrary number, a task that may be easily performed by marginally  trained technicians.
Latent print identification is more than simply counting ridge characteristics:  it involves many factors, including the skill gained only through experience.  Latent print identification is a visual information--processing system employing scientific methodologies and human problem--solving techniques and requiring considerable experience for their proper employment.  Experienced examiners, consciously or unconsciously, will consider, in addition to the number of characteristics, the overall quality and clarity of the impression, the rarity of the pattern type or ridge flow, and the uniqueness of the ridge characteristics.
Above all, the experienced examiner knows that the validity of the identification can be demonstrated to the satisfaction of other qualified examiners."7

“There is no magic number that determines a positive ”make" or identification."
“A specific, absolute number of points of comparison, or matching characteristics, should not be paramount.  Is the acknowledged expert in his field totally satisfied he has made an identification, irrespective of the number of matching characteristics in the two prints?”
“The final decision as to identification should rest with the experienced latent print expert, subject to verification by equally qualified latent print experts.”8

“Where an examiner is, by mandate or preference, limited to reporting results by minimum numerical standard or concurrence rules, certain ethical and legal problems may arise.  In the courts of the United States, and in many other countries, the final task of the examiner is to testify as an expert to his or her own opinion.”9

“The notorious ”12 point rule" in fingerprint identification is not based on scientific studies of friction skin characteristics, their type (shape) and their frequency of occurrence."
“The effect of ”number" rules in fingerprint identification is to make the identification process a rote function.  Such rules discourage use of other valuable identifying information.  “number” rules also discourage (prohibit) the expert from exercising expert abilities."10

Endnotes

1  Soderman & O'Connell, MODERN CRIMINAL INVESTIGATION, P. 146, Funk & Wagnells, 1952!!.

2  Olsen, Robert D. SR., SCOTT'S FINGERPRINT MECHANICS, P.28, Thomas, 1978.

3  Ashbaugh, David, RIDGEOLOGY, MODERN EVALUATIVE FRICTION RIDGE IDENTIFICATION, P. 29--30,  Royal Canadian Mounted Police.

4  Grieve, David, THE IDENTIFICATION PROCESS:  ATTITUDE AND APPROACH, P.216--217, Journal of Forensic Identification, 39(5), 1988.

5  Federal Bureau of Investigation, AN ANALYSIS OF STANDARDS IN FINGERPRINT IDENTIFICATION, FBI Law Enforcement Bulletin, June, 1972.

6  International Association for Identification, REPORT OF THE STANDARDIZATION COMMITTEE, P. 11--16, Fingerprint and Identification Magazine 55(4), 1973.

7  Lee & Gaensslen, ADVANCES IN FINGERPRINT TECHNOLOGY, P. 55--56, Elsevier, New York, 1991.

8  Clements, Wendell, THE STUDY OF LATENT FINGERPRINTS -- A SCIENCE, P. 109--122, Thomas, Springfield, Ill., 1987.

9  Cowger, James, FRICTION RIDGE SKIN, COMPARISON AND IDENTIFICATION OF FINGERPRINTS, P. 146, Elsevier, New York, 1983.

10  Phillips, Clarence, THE FINGERPRINT IDENTIFICATION PROCESS, Speakers Abstract, International Association for Identification, California State Division, 77th Annual Educational Conference, May 1993.

(Editor — Many debates over the years have stemmed from the issue of a set minimum number of  characteristics.  And the debates will probably continue long after we are gone.  I would offer that discussions of this subject are healthy exercises — as they prepare an examiner to offer explanations in court.
  However, the issue is far from  settled (as some have suggested) — unless they accept the Identification Community Standard as offered in the references above.  The “settled” discussion can only be claimed by the authorative reference of a recognized body,  such as  endnote 6 above.
When one stands on a particular side of an issue, one should also look at the company in which they stand or in the words of Miguel De Cervantes “Tell me thy company, and I'll tell thee what thou art.” And I  would ask with whom do you stand?)

This article was originally published in “THE PRINT” 10(3), March 1994, pp 1-3
and has been obtained from the online library provided by the
Southern California Association of Fingerprint Officers
www.scafo.org