Interpreting DNA under fingernails given activity level propositions.

Abstract

We have generated a general lookup table of LRs that could be used in the general case of a person of interest (POI) accused of attacking a complainant and the evidence is DNA from fingernails of the complainant. We consider that social interaction may have occurred between the complainant and the POI or others, comments from the complainant about scratching the attacker (or not), and visible scratches on the POI (or not). The LRs are generally small, ranging from a few thousand to less than one. If POI and no other DNA (except the complainant (C)) are present with no opportunity for social interaction between C and POI, then the LR is assigned a value in the thousands. The evidence supports the H_p proposition (LR between 3 and 10) more than the alternative if there has been socialization between C and POI, and as long as there is no unknown DNA observed. The presence of unknown DNA as well as POI DNA gives LRs from 500 to 700 in the absence of social contact between C and POI. The evidence is neutral if neither POI nor unknown DNA is present. DNA analysis of fingernails is exculpatory if the POI is excluded from the findings and unknown DNA is present. Considering whether the complainant claims to have scratched her attacker increases the discrimination power of the model by a small amount, increasing the support for H_p when only POI DNA is present, or increasing the support for H_a if only unknown DNA is present.