AFTE Theory of Identification
In 1985, the Criteria for Identification Committee formalized the AFTE Theory of Identification as it Relates to Toolmarks. The theory articulates three principles that provide the conceptual basis for comparing toolmarks for the purpose of identifying them as having a common source.
For the purpose of this module, consider any reference to the word tool in this section as a reference to the machined surfaces of the action and mechanism of a firearm. Any reference to the term toolmark refers to striated or impressed marks on a fired cartridge case or unfired cartridge cycled through the action of a firearm.
The three principles of the AFTE Theory of Identification as it Relates to Toolmarks:
- The theory of identification as it pertains to toolmarks enables opinions of common origin to be made when the unique surface contours of two toolmarks are in sufficient agreement.
- This sufficient agreement is related to the significant duplication of random toolmarks as evidenced by the correspondence of a pattern or combination of patterns of surface contours. Significance is determined by the comparative examination of two or more sets of surface contour patterns comprised of individual peaks, ridges, and furrows. Specifically, the relative height or depth, width, curvature and spatial relationship of the individual peaks, ridges and furrows within one set of surface contours are defined and compared to the corresponding features in the second set of contours. Agreement is significant when it exceeds the best agreement demonstrated between two toolmarks known to have been produced by different tools and is consistent with agreement demonstrated by toolmarks known to have been produced by the same tool. The statement that sufficient agreement exists between two toolmarks means that the likelihood another tool could have made the mark is so remote as to be considered a practical impossibility.
- The current interpretation of individualization/identification is subjective in nature, founded on scientific principles and based on the examiners training and experience.
Sufficient Agreement and Best Known Nonmatch
The second of the three principles of the AFTE Theory of Identification indicates that the degree of correspondence which must be exceeded to constitute sufficient agreement for an identification is the best known nonmatch (by each individual examiner) to have been produced by different tools. Ideally, the examiner would gain experience in this during their initial training period rather than when they begin to perform actual examinations on their own.
The third principle of the AFTE Theory of Identification indicates that, although founded on the scientific method and reproducibility of results, the interpretation is subjective in nature. It is the policy of most laboratories that a second qualified examiner verify the findings of the first examiner.
Ultimately, sufficient agreement is the product of the examiners personal training, skills, and experience in
- recognizing corresponding patterns of matching striations,
- recognizing corresponding patterns within impressed toolmarks,
- determining the best known non-match in their personal experience,
- training and experience in comparing striated and impressed toolmarks.
It is incumbent on each examiner to rely on their training and experience to identify and to be able to articulate the process used to determine sufficient agreement and best known nonmatch.
AFTE Range of Conclusions
For purposes of this module, typical fired cartridge case and shotshell case comparisons fall into two broad categories:
- An evidence cartridge case or shotshell case from a crime scene is identified as having been fired in a particular evidence firearm.
- Evidence cartridge cases or shotshell cases recovered from the same or separate incidents are identified as having been fired in a single firearm (no firearm submitted).
Based on the AFTE Theory of Identification, there are four categories of examination outcomes (Range of Conclusions Possible When Comparing Toolmarks) typically used by firearm examiners in the microscopic comparison of fired cartridge cases and shotshell cases.
These categories are accepted for all types of toolmark comparisons:
- Unsuitable for comparison
The AFTE Glossary defines an identification as follows:
"Agreement of a combination of individual characteristics and all discernable class characteristics where the extent of agreement exceeds that which can occur in the comparison of toolmarks made by different tools and is consistent with the agreement demonstrated by toolmarks known to have been produced by the same tool."
This statement reflects the concepts of sufficient agreement and best known nonmatch. All identifications are based on pattern matching. It is possible to go beyond this qualitative match to the quantifiable consecutive matching striae (CMS) approach to further support an identification.
An inconclusive result is noted as the outcome of a comparison in which there is
- some agreement of individual characteristics and all discernable class characteristics, but insufficient for identification,
- agreement of all discernable class characteristics without agreement or disagreement of individual characteristics due to an absence, insufficiency, or lack of reproducibility,
- agreement of all discernable class characteristics and disagreement of individual characteristics, but insufficient for an elimination.
As defined in the AFTE Glossary , elimination is a significant disagreement of discernable class characteristics and/or individual characteristics. For purposes of fired cartridge case and shotshell case comparisons, an elimination is most often based on observed differences in any class characteristic.
However, an elimination based on individual characteristics is more complex. Conceptually, an elimination based on individual characteristics means that if a firearm can be shown to have never been subjected to significant use or abuse over a period of time, the qualitative aspects of the striations produced on fired cartridge cases and shotshell cases should remain the same. A difference in these qualities indicates an elimination.
Elimination based on individual characteristics requires a detailed knowledge of the history and treatment of the firearm, as well as documentation to support the history. It is the responsibility of the examiner to provide this historical documentation. This type of elimination should be approached with caution. Many experienced examiners have never made such an elimination, and the protocols of many laboratories do not allow it.
Eliminations are as sensitive and important as identifications. When eliminations based on microscopic marks alone are permitted by laboratory protocols, the process should adhere strictly to the criteria for an elimination set out in those protocols. When a firearm is included in the evidence and an elimination is based solely on individual characteristics, examiners may add phraseology to their report indicating that their conclusion was based on the condition of the firearm as received or the firearm in its current state of assembly.
The AFTE Glossary designates this category as unsuitable for comparison. This outcome is appropriate for fired and mutilated cartridge cases and shotshell cases that do not bear microscopic marks of value for comparison purposes.
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