Examination of Firearms Review: 2007 to 2010 - Interpol

ullet of the same type. This involves the subdivision of the impression marks into
a series of individual lines with width equals to the separation distance at which a
misalignment of striations between the bullets is indistinguishable. This distance
depends on the microscopic resolution limits and the visual acuity of the examiner.
Calculated probabilities for finding pairs and triplets of consecutively matching
striations on non-matching bullets, by an examiner with normal eyesight using a
microscope at 40X magnification, gives values that are in good agreement with the
empirical probabilities determined in the 1950s, and when determined for larger
consecutive sequences indicate that they are very unlikely to occur. These
formulae can be used to calculate the probabilities for the random occurrence of
any sequence of striations, providing a means by which the significance of a
specific match between any two bullets can be justified quantitatively.
The error rates of any given scientific evidence form today a substantial element in
the admissibility of this evidence in court. Murphy (109) addressed this issue in a
presentation available at the SWGGUN Admissibility Resource Kit (ARK), based
on the Collaborative Testing Services (CTS) firearm and toolmark examination
tests (1992-2005). Despite the views expressed by the tests’ provider, CTS, that
“CTS Summary Reports should not be used to determine forensic science
discipline error rates…” (110), Murphy analyses the data anonymously published
in the CTS final reports and presented the calculated false-positive and falsenegative
rates, as well as the sensitivity and the specificity of both firearm
identification and toolmarks examination. The calculated error rate for the
toolmarks proficiency tests are as follows: False positive (wrong identification) rate
– 1.7%, false negative (wrong exclusion) rate – 1.6%, sensitivity (the ability to
detect the right identification) – 90.6% and specificity (the ability to detect the right
exclusion) – 57.9%. Since one of the objections for using the CTS reports for error
rates evaluation is that not all of the participants of these tests are qualified
experts, Murphy presents also the error rates when the trainees removed, and find
them to be even lower.
3.7 Miscellaneous Issues
Swanepoel (111) presented an interesting and rarely-published case where unique
dual-impression encountered during the comparison of a stolen hydraulic pump
and the base plate from which the pump was allegedly stolen. There was an
agreement of class characteristics and sufficient agreement of individual
characteristics which were of such significance that it could be concluded that the
hydraulic pump and the base plate were at one time connected to each other.
The forensic analysis of knot evidence is an uncommon examination type.
Nevertheless, Chisnall published several articles in this field, dealing with the
strength and limitations of such analysis, with tying anomalies and their
significance in analysing knot evidence and with tying habits (112-114). This
author describes how properly preserved and analysed knot evidence can offer
corroborating information, indicate leads to other sources of evidence. A survey of
562 volunteers, conducted by Chisnall, revealed that only a minority ties
noteworthy anomalies that differ from the common trend. Finding these rare
anomalies in crime scenes might have significant evidential value. If similar
72