Examination of Firearms Review: 2007 to 2010 - Interpol

quantifiable measures of reliability and accuracy, quantifiable measures of
uncertainty in the conclusions, and the lack of research on observer bias and sources
of human error. DNA analysis however has proven to give consistent results, with a
high degree of certainty, sufficient to demonstrate the connection between evidence
and a specific individual or source. In this respect the stochastic effects observed
during PCR and especially Low Copy Number typing (LCN) can complicate the
procedure. To make no mistakes, or have a zero error rate is practically impossible.
Therefore it should be a good scientific practice to show the limits of a specific
method and to address for example the impact of potential bias. Thompson 41 gives
an example of a blind procedure, preventing observer bias, in forensic DNA profile
interpretations. He first argues that “the widespread failure to adopt blind procedures
for making DNA interpretations sets the stage for target shifting”. In which target
shifting means that examiners change their interpretation on the basis of the
available reference profiles from suspects which they know are likely to match.
Thompson 41 proposes to prevent this by sequential unmasking, in which the features
of the contested material are observed first. After this the examiner has to point out
which DNA profiles could contribute to this material. Only after that he can continue
his investigation, comparing the reference samples with the contested material.
As the DNA technology is advancing, the statistical part becomes more and more
important. Hedman et al 42 proposed a statistical model for unbiased quality control of
forensic DNA profiles (e.g. capillary electrophoresis electropherograms). The
measure proposed is the forensic DNA profile index (FI). Where the total peak height
of the capillary electrophoresis electropherograms (e.g. the sum of the heights of the
observed STR peaks given in relative fluorescent units (rfu)) together with the mean
local balance (e.g. the mean of intra-locus balances or discrepancies between peak
heights within a heterozygous STR marker), and the Shannon entropy (e.g.
discrepancies, or inter-locus balance, between the sum of the peak heights between
the markers) 42 are combined. The FI can be used to ensure the overall quality of
DNA profiling within a laboratory.
5.2 Low Copy Number
In order to analyze samples that contain less than 200 pg of template DNA 28,43 (in
literature referred to as low template DNA (LTDNA) or touch DNA) LCN can be used.
There are several variants of the LCN technique, the most basic (the variant with 34
PCR cycles, a replication strategy with two or more replications, and a validated
interpretation methodology) is used in the Netherlands, the UK and in New
Zealand 44 . LCN uses STRs to produce a DNA profile. In comparison to the
conventional STR methods applied in normal casework LCN uses methodologies to
increase DNA detection sensitivity. For example modifications to the PCR (e.g. other
primers) and/or post-PCR manipulations 34,28,43,45 .
The increase in detection sensitivity gives a concomitant increased risk of
contamination. For example more sensitivity of detecting DNA, enhances the chance
to find not only the DNA profile of the perpetrator, but also the DNA profile of other
persons 46 (whose cells were present on e.g. the victim due to casual contact 28 ).
Furthermore stochastic effects are also more likely to occur. Stochastic effects are
heterozygote peak imbalance, allele or locus drop-out, and stutter (for more
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