Annual Conference 2021, ACC Liverpool
Forensic comparison of insole footprint marks:
A case study of the overlay method
Nick Howsam, Podiatrist and Postgraduate Researcher for PhD in Forensic Podiatry, Private Practice and University of Huddersfield
- The overlay method is a common approach used not just for forensic podiatry, but in other forensic science disciplines to compare evidence. Literature reflects the subjective nature and weak scientific rigour of this method for comparing features of insole footprint marks from crime scene and suspects’ footwear.
- Strength of agreement among three practising forensic podiatrists’ overlay comparisons of ten unidentified insoles showed two had high inter-rater consistency between their comparative conclusions (Rater1 v Rater2 - Kappa: 0.981; 95%CI: 0.943 to 1.020) whilst one did not (Rater1 v Rater3 - Kappa: 0.340; 95%CI: 0.148 to 0.532; Rater2 v Rater3 - Kappa: 0.310; 95%CI: 0.100 to 0.519).
- Data showed a method accuracy of 76.2% for all overlay comparisons, with ‘Moderate Evidence to Support’ to be the most ideal level of conclusion in the Verbal Expressions Table to decide between compatibility or incompatibility of comparisons (Validity Parameters: Sensitivity: 66.7%; Specificity: 90.5%; False Positive Rate: 9.5%; False Negative Rate: 33.3%).
- Results provide initial evidence of the scientific foundation of the overlay method in a forensic podiatric context and support the view that this method should not be used in isolation to compare insole footprint marks when used to assist in the process of forensic human identification.