A large-scale statistical analysis of barefoot impressions

• Published in: Journal of forensic sciences Vol. 50; no. 5; p. 1071
• Main Authors: Kennedy, Robert B, Chen, Sanping, Pressman, Irwin S, Yamashita, A Brian, Pressman, Ari E
• Format: Journal Article
• Language: English
• Published: United States 01.09.2005
• Subjects: Adult; Analysis of Variance; Female; Foot - anatomy & histology; Forensic Medicine - methods; Humans; Image Processing, Computer-Assisted; Male; Models, Biological; Principal Component Analysis; Probability
• ISSN: 0022-1198
• DOI: 10.1520/JFS2004277

In an earlier paper, outlines of footprints of persons walking normally were studied to determine whether different people make verifiably distinct footprints. Our basic null hypothesis is: given a footprint outline trace made by Subject A (Alice), then Subject B (Bob), a distinct person, cannot produce a footprint outline trace indistinguishable from that of Alice. We showed in the previous work that the probability of a chance match is less than 10(-8). In this paper we report two new advances in our research. First, we establish a rigorous mathematical framework for calculating worstcase and average chance-match probabilities. Second, we repeat the previous experiment to substantiate the earlier results, but with an expanded population sample size and a more representative and significantly bigger repeated sample. These improvements and a new automated tracing procedure for extracting all numerical measures lead to a sharpened accuracy with average chance match probabilities of 7.88 x 10-(10) for a general population. In other words, the odds of a chance match are one in 1.27 billion.