Reporting given activity level propositions is progressing from professional opinion to more data driven approaches. Data are needed to infer probabilities of recovery of DNA or its biological matrix through transfer, persistence, prevalence and background. Such data may be obtained from scientific literature, casefile studies, and case specific experimentation. The currently available sources of data are rich and diverse and have generally proved a good foundation for such evaluative opinions.

The translation of the data from generic experiments to the highly specific circumstances of a case however requires subjective assessments and assumptions. For instance, assessing how close the experimental conditions in a study align with the alleged activities in the case scenarios, or assuming that the selection of volunteers involved in an experiment will demonstrate a similarity in crucial characteristics – like shedding propensity – to that of the persons-of-interest in a case. If there is no reason, based on case information or scientific data, to assume otherwise, such assumptions may be considered reasonable to infer probabilities of recovery of DNA through DNA transfer and persistence.

The probability of recovery of background or prevalent DNA from surfaces however is highly specific to the circumstances of a case and the life history of items. The relation of a person-of-interest to an item or location may be very specific in frequency and nature. Commonly, information about these specifics will be unknown or unavailable to the scientist, making a reasonable selection of relevant data complex. In a recent publication [1] we therefore recommended the collection and use of contextual samples to inform probabilities of recovering prevalent or background DNA.

In this presentation we briefly reiterate the concept of contextual samples. We will demonstrate their use in a case example. We will subsequently explore the impact of contextual samples on the likelihood ratio through sensitivity analyses, demonstrating their added value in specific situations.

Authors

• Y.R. Goedhart (Amsterdam University of Applied Sciences, Netherlands)
• J.A. De Koeijer (Netherlands Forensic Institute, Netherlands)
• L.H.J. Aarts (Netherlands Forensic Institute, Netherlands)
• C.J. De Poot (Amsterdam University of Applied Sciences, Netherlands)
• B. kokshoorn (Netherlands Forensic Institute, Netherlands)

Reference

[1] Goedhart, Y. R., de Koeijer, J. A., Aarts, L. H., de Poot, C. J., & Kokshoorn, B. (2026). Contextual DNA samples in evaluations given activity level propositions. Forensic Science International: Genetics, 84, 103452.