Accounting for inter-laboratory DNA recovery data variability when performing evaluations given activities

Abstract

A large, recently published, inter-laboratory study by the ReAct group has shown that there is considerable variability in DNA recovery that exists between forensic laboratories. The presence of this inter-laboratory variability presents issues when one laboratory wishes to carry out an evaluation and needs to use the data produced by another laboratory. One option proposed by the ReAct group is for laboratories to carry out a calibration exercise so that appropriate adjustments between laboratories can be made. This will address some issues, but leave others unanswered, such as how to make use of the decades of transfer and persistence data that has already been published. In this work we present a method to utilise data produced in other laboratories (whether it provides DNA amounts or a probability of transfer) that takes into account inter-laboratory variability within an evaluation. This will allow evaluations to continue, without calibration data, and ensures that the strength of findings is appropriately represented. In this paper we discuss complicating factors with the various ways in which previous data has been reported, and their limitations in supporting probability assignments when carrying out an evaluation. We show that a combination of producing calibration information for new data (as suggested by the ReAct group) and development of strategies where calibration data is not available will provide the best way forward in the field of evaluations given activities.