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"Factor of Two" Issue in Mixed DNA Profiles

NCJ Number
Date Published
December 2014
7 pages
This study explored how the likelihood ratio (LR) for any mixture can be constructed to report correctly at the sub-source level by taking contributor orders and genotype set orders into account; worked examples of the LR calculation are included to assist in explaining this confusing issue.
A commonly used idea in forensic fields is known as the ‘hierarchy of propositions’. DNA analysts commonly report at the sub-source level in the hierarchy. This means that they simply comment on the probability of the evidence for the given propositions that consider contributors that lead to a DNA profile and not on the source of specific biological components, not the activity that led to the transfer or the offense that is reported to have occurred; however, DNA analysts also commonly report at a level even lower than the sub-source level. In this ‘sub–sub-source’ level only reference comparisons to components of a mixture are reported. The difference between the sub-source level and sub–sub-source level is the difference between comparing an individual to a mixture as a whole, or comparing them to only one component of a mixture. This idea has been expressed in the past as the ‘two trace’ problem or the ‘factor of two’ problem. With the advent of expert systems that can provide a measure of weight of evidence in the form of a likelihood ratio (LR) for any mixture, resolvable or not, the distinction between these two levels becomes more important. (Publisher abstract modified)

Date Published: December 1, 2014