EXPERT  OPINION � FACT  OR  FICTION?
RESPONSIBILITIES  OF  THE  EXPERT WITNESS

(The following paper was presented at the February  12, 1994 SCAFO meeting)

By  WAYNE  PLUMTREE
Supervising Criminalist
Los Angeles County Sheriff's Department
Scientific Services Bureau--� Identification Section

The Evidence Code (Sec. 720) defines an expert witness as an individual �...[that] has special knowledge, skill experience, training, or education sufficient to qualify him as an expert on the  subject ...�  This is a very broad and circular definition covering a wide range of expertise and opinions.  As in any field (checkers to nuclear physics) the training, experience and knowledge possessed and offered by fingerprint experts varies widely.  

Q.  What does the latent print expert offer?  

A.  The expert offers interpretations, based upon scientific principals, that are reproducible.

The field of latent print identification is well established and a consensus by experienced latent print examiners is readily obtained on �makes� and �eliminations.�  The two scientific premises upon which latent print comparison is based --� friction ridges on the human body remain the same through--out the life of an individual and no two fingerprints are identical --� are easily verified by observation and have been well documented.  The only areas really open for interpretation are: a) the number of characteristics that are necessary to uniquely identify a given print and b) interpretation of a developed latent (i.e.: what characteristics are observable).  Thus the �opinions� offered by latent print experts are not mere �opinions;� but are more an interpretation or conclusion by trained individuals after conducting an examination employing scientific principals.

BASIS  OF  FORENSIC
COMPARISON OPINIONS

In order to make a comparison it is necessary to distinguish between what characteristics will result in identification and what will not.  CLASS characteristics are not unique to a given object, while INDIVIDUAL characteristics are unique.  Whatever is being compared must possess class and/or individual characteristics.  Diagram #1 demonstrates class vs. individual characteristics, diagram #2 gives examples of class and/or individual while diagram #3 applies class and individual characteristics to latent prints.

Evidence possessing individual characteristics only:
In addition to friction ridge comparison, there exists only three types of forensic examinations that can result in conclusive matching: tool mark comparison, impression comparison (shoe prints, tire tracks, etc.), and the physical fit.  DNA blood typing is not individual as only a portion of the available DNA is being examined and even if DNA typing becomes individual, identical twins would have the same DNA blood type.  In each of these examinations (tool marks, shoe prints, physical fit & fingerprints), with sufficient individual characteristics, a trained examiner can determine if there is a unique connection between the questioned and reference material.  The possible results of the examination are shown in Diagram #4 and are �make (match),� �inconclusive,� or �elimination.�

Evidence possessing class characteristics only:
There is a multitude of evidence that only possesses class characteristics such as hair, fibers, gasoline, blood antigens, blood enzymes, paint metal, etc.  The only definitive conclusion that can be formulated is an �elimination.�  The strongest conclusion demonstrating a positive relationship is limited to �possibly� or �probably.�  And if no conclusion can be formed, the answer is  �inconclusive.�  See diagram #5.

Evidence possessing both class and individual characteristics:
Evidence from the first group discussed having individual characteristics may also have class characteristics.  When both are present there are four possible conclusions: �make,� �possibly,� �inconclusive� and �elimination.�  See diagram # 6.

LATENT PRINT EXPERT TESTIMONY

Latent print evidence:
A vast majority of latent print examiners use class characteristics to eliminate prints and use only individual characteristics when conducting a comparison.  The three categories that can be used when individual characteristics only are being compared are �make,� �inconclusive� and �elimination.�  The term �no make� (at our facility) includes both �inconclusive� and �elimination� and is used when the examiner did not conduct an �elimination� examination.  See diagram # 7.

Probably is an acceptable opinion in several forensic disciplines.  However, in friction ridge comparisons, the offering of an opinion of �probably or possibly� is not acceptable without qualifying or fully explaining the opinion1.  This standard is thought to have been set forth to protect this most highly respected discipline.  Realizing the ease in which a probable or possible   conclusion could be misinterpreted as having more significance than the evidence supports demands the strict adherence to the offering of only acceptable latent print opinions (make, inconclusive or elimination).

Latent print expertise:
Each expert has a different level of experience, education, and experience.  The more experienced examiner may be able to locate and identify individual characteristics on a developed latent print that would be missed by a novice; possibly resulting in the experienced examiner providing a definitive opinion (make or elimination) with the novice giving an inconclusive result.  The more experienced examiner may make an identification with fewer characteristics (Galton) than would a less experienced person; resulting in a �make� for the experienced examiner, while the less experienced individual would have to call the print exam �inconclusive.�  A correct conclusion of �inconclusive� by one examiner does not eliminate the possibility that another examiner may correctly conclude that the comparison was a �make.�  This is due to differences in skill and observation skills of the examiners.  But a correct conclusion of �make� by one examiner precludes a correct conclusion of �elimination� by another examiner and visa versa, see diagram #8.

In summary the following conclusions are  possible with both examiners being correct:

Examiner  A Examiner  B
make  inconclusive
make make
 inconclusive make
 inconclusive elimination
elimination inconclusive
elimination elimination

The following conclusions are inconsistent and require one examiner to be incorrect:

Examiner  A   Examiner  B
make elimination
elimination make

An internal conflict like this must be resolved.  The supervisor is obligated to take action and release only a correct conclusion to the investigators or court.  Should a disagreement of this type arise in court (between different agencies), the incident must be investigated by organizations competent to handle these technical issues.  If left unchecked, juries can be given inaccurate or misleading information.

In view of the bad publicity sometimes directed at us because of less than honest or qualified print examiners, it is imperative that we as an organization of individuals interested in furthering the science of fingerprint comparison keep our house in order and, as experts, exercise proper responsibilities as the credibility of the science is at stake.

Endnote:
1International Association for Identification, Article 5.

(Editor--� Mr. Plumtree has a Bachelor of Science Degree in Chemistry, and a Master's in Criminalistics.    As a criminalist for over 22 years with the Los Angeles County Sheriff's Department, he has worked and gained expertise in various sections and forensic disciplines, including: the laboratory's Physical Section, the Serology Section, and the Firearms Section, where he was the Supervising Criminalist. While he is not a latent print examiner by training, as the supervisor of the Identification Section for the last four years, he has gained a tremendous understanding and insight into latent print examinations.  With his broad forensic background, he offers these observations as to the science of friction skin identification.)

This article was originally published in �THE PRINT� 10(2), February 1994, pp 3-6
and has been obtained from the online library provided by the

Southern California Association of Fingerprint Officers
www.scafo.org