Highlights
- •We report the distribution of the number of contributors conditioned on context.
- •Conditioning on an assumed contributor can shift the distribution to a larger n.
- •The degree of change depends on whether the assumed contributor is major or minor.
- •Computing a LR treating the NOC as a nuisance variable requires consistent context.
Abstract
Forensic DNA signal is notoriously challenging to assess, requiring computational
tools to support its interpretation. Over-expressions of stutter, allele drop-out,
allele drop-in, degradation, differential degradation, and the like, make forensic
DNA profiles too complicated to evaluate by manual methods. In response, computational
tools that make point estimates on the Number of Contributors (NOC) to a sample have
been developed, as have Bayesian methods that evaluate an A Posteriori Probability
(APP) distribution on the NOC. In cases where an overly narrow NOC range is assumed,
the downstream strength of evidence may be incomplete insofar as the evidence is evaluated
with an inadequate set of propositions.
In the current paper, we extend previous work on NOCIt, a Bayesian method that determines
an APP on the NOC given an electropherogram, by reporting on an implementation where
the user can add assumed contributors. NOCIt is a continuous system that incorporates
models of peak height (including degradation and differential degradation), forward
and reverse stutter, noise, and allelic drop-out, while being cognizant of allele
frequencies in a reference population. When conditioned on a known contributor, we
found that the mode of the APP distribution can shift to one greater when compared
with the circumstance where no known contributor is assumed, and that occurred most
often when the assumed contributor was the minor constituent to the mixture.
In a development of a result of Slooten and Caliebe (FSI:G, 2018) that, under suitable
assumptions, establishes the NOC can be treated as a nuisance variable in the computation
of a likelihood ratio between the prosecution and defense hypotheses, we show that
this computation must not only use coincident models, but also coincident contextual
information. The results reported here, therefore, illustrate the power of modern
probabilistic systems to assess full weights-of-evidence, and to provide information
on reasonable NOC ranges across multiple contexts.
Keywords
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Article info
Publication history
Published online: July 07, 2021
Accepted:
July 3,
2021
Received in revised form:
June 24,
2021
Received:
April 20,
2021
Identification
Copyright
© 2021 Elsevier B.V. All rights reserved.
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- Corrigendum to “The a posteriori probability of the number of contributors when conditioned on an assumed contributor” [Forensic Sci. Int. Genet. 54 (2021) 102563]Forensic Science International: GeneticsVol. 58
