Probabilistic DNA Evidence: The Layperson’s Interpretation

Jessica Ritchie

    Research output: Contribution to JournalArticlepeer-review


    A recent Australian High Court case held it was acceptable to express DNA results as either a frequency ratio or as an exclusion percentage. In order to understand if these two approaches could affect the outcome of a criminal trial, this study collected online survey data from the general public who were eligible for jury duty in Australia (n = 258). Participants were randomly assigned and completed two vignettes with two different forensic results that were manipulated in a 2×2 between-group design. Results found the way evidence was presented was sometimes statistically significant on the verdict in the case, and when not, the relationship was going in the predicted direction. Specifically when evidence was presented as an exclusion percentage, participants were more likely to convict than when presented with frequency ratio evidence. This is important as research suggests that once DNA evidence is admitted the effect can be difficult to undo, even with extensive cross-examination and testimony. DNA is a valuable tool for the criminal justice system; however, this study considers whether there is a need for standardisati
    Original languageEnglish
    Article number47
    Pages (from-to)440-449
    Number of pages10
    JournalAustralian Journal of Forensic Science
    Issue number4
    Early online date19 Jan 2015
    Publication statusE-pub ahead of print - 19 Jan 2015


    • DNA evidence
    • jury decision-making
    • exemplar cueing theory
    • probabilistic reasoning
    • forensic science


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