Does the use of probabilistic genotyping change the way we should view sub-threshold data?

The sensitivity and resolution of modern DNA profiling hardware is such that forensic laboratories generate more data than they have resources to analyse. One coping mechanism is to set a threshold, above the minimum required by instrument noise, so that weak peaks are screened out. In binary interpretations of forensic profiles, the impact of this threshold (sometimes called an analytical threshold - AT) was minimal as interpretations were often limited to a clear major component. With the introduction of continuous typing systems, the interpretation of weak minor components of mixed DNA profiles is possible and consequently the consideration of peaks just above or just below the analytical threshold becomes relevant. We investigate here the occurrence of low-level DNA profile information, specifically that which falls below the analytical threshold. We investigate how it can be dealt with and the consequences of each choice in the framework of continuous DNA profile interpretation systems. Where appropriate we illustrate how these can be implemented using the probabilistic interpretation software STRmix. We demonstrate a feature of STRmix that allows the analyst to guide the software, using human observation that there is a low-level contributor present, through user-designated prior distributions for contributor mixture proportions.