Examination of Firearms Review: 2007 to 2010 - Interpol

3.4 Examination of Consecutively-Manufactured Tools
One of the ways of demonstrating the uniqueness of toolmarks is by studying the
marks produced by consecutively-manufactured tools or firearms. Buckleton and
his colleagues (84) suggested an experimental design for acquiring relevant data
for addressing this issue. The authors accept the fundamental soundness of this
approach, and do not suggest that previous work was unsound, but urge the
toolmarks examiners’ community to adopt several modifications in the
experimental procedure: “Blinding” the test, distribute the exhibits to the examiners
randomly, and publish all the results . The proposed experimental design is a lot
more time consuming than the usual experiment, however these authors feel that
many of the minor faults in current experiments would be eliminated by this
approach.
An example for an empirical study for the validation of toolmarks examination was
published by Giroux (85). Five consecutively-manufactured flat-bladed
screwdrivers were acquired from the manufacturer, and toolmarks were produced
on sheet-lead test material, in a uniform manner, using both sides of each tool.
The test samples were then randomly divided into test-sets (kits) and examined by
eight qualified toolmarks examiners (each received, randomly, one known mark
and 10 questioned ones). As published, there were no mis-identifications in this
study – 0% false positive error rate (0/51). However, there was one mis-elimination
– 3.4% false negative error rate (1/29). This mis-elimination calls for attention to
the topic of elimination criteria, especially when the questioned tool is not available
for examination.
Another study regarding the toolmarks produced by consecutively-manufactured
screwdriver tips was conducted by Chumbley, Faden and co-authors (86-88), as
part of a project on the development of a non-destructive optical profilometry
method for toolmark examination. The profiles obtained were compared and
statistically evaluated using an algorithm developed for comparing twodimensional
images of toolmarks. According to these authors, their results
demonstrate that screwdriver tips are unique in surface profile, not only from tip to
tip, but from side to side of each tip as well.
As part of research into the validation of this analysis, initiated by relevant US
court decisions (including, for instance, Ramirez vs. State of Florida, No. 66,992,
1989), Lancon (89) examined marks made in bone using ten consecutively
manufactured knives. The test cuts in the bone (pig ribs) and the reference cuts
were all recorded using silicone-based casting material (Microsil®), and compared
using a comparison microscope. Eventually, all of the blades could be matched to
their corresponding cuts. Representatives of the best known non-match (KNM)
comparisons were photomicrographed for further reference. According to the test
results, bone is a suitable material for accepting toolmarks, and consecutively
manufactured knife blades can be differentiated using conventional techniques
used by firearm and toolmark examiners.
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