Examination of Firearms Review: 2007 to 2010 - Interpol

anomalies can be found in suspect samples, the link between the suspect and the
case knots is stronger. In addition, the study shows that right-handed subjects
tended to tie S-twisting hitches more often than Z-twisting hitches (S- and Ztwisting
refer to the direction of one strand over the other) while left-handed ones
demonstrated S tying less frequently and even tied Z knots more often in some
situations.
Desiderio and Chin (115) stress the need for a more synergistic approach for
toolmark examinations, and are discussing the important role that trace evidence
(like physical match, paint or DNA) plays during the examination of toolmarkrelated
evidence. In order to illustrate the significance of the relationship between
trace evidence and toolmarks, being complementary to each other, various case
related studies are presented.
4. Physical Match
Physical match, namely linking two or more objects by the morphology of fractured
or torn surfaces, is usually viewed as one of the strongest ways for establishing
common origin. The evidential value of such physical matches, and their
admissibility in court, seem to be taken for granted, considering the limited number
of articles published during the Review period in this discipline.
Christensen and Sylvester (116) conducted a validation study for the reliability of
physically matching fragments of bone and other mineral-based biological
materials such as shells and teeth. Participants with varying education, training
and experience were asked to complete a matching exercise consisting of
intentionally fragmented specimens. Success rates were very high - the positive
association (correct match) rate was 0.925, while the non-association (overlooked
match) rate was 0.075, and negative associations (incorrect matches) occurred at
a rate of just 0.001. Results also indicate that participants with more education and
related experience tended to have higher positive association rates, although not
significant statistically. Experienced osteologists, however, completed the
matching exercise in significantly less time. Low error rates among both
experienced and inexperienced individuals support the reliability and validity of
performing physical matches of these materials, and suggest that performance
may also be related to an individual's aptitude for spatial tasks or other factors.
In a series of publications, De-Smet and his colleagues (117, 118) presented two
dimensions (2D) and three dimensions (3D) fracture matching of snap-off cutter
blades, using numerical algorithms and surface area based reliability evaluations.
Like other researchers, these authors used controlled breaking conditions, using a
materials testing machine, for producing the test samples. The use of a
commercial white-light profilometer system for obtaining 2D and 3D image surface
scans of multiple fractured objects is discussed, and the results showed that this
approach performs quite well, confirming all matches in the test-set and passing a
blind test. The conclusions of this project were that 3D profilometry can aid in
developing more objective methods for examining fracture matching surfaces, that
evaluation of automated methods parameters may be a required step towards
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