Effect of frame rate on the ability to identify characteristics of gait from CTV

NewsBot · Nov 24, 2013

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The effect of frame rate on the ability of experienced gait analysts to identify characteristics of gait from closed circuit television footage
Ivan Birch, Wesley Vernon, Gordon Burrow, Jeremy Walker
Science & Justice; Article in Press

Forensic gait analysis is increasingly being used as part of criminal investigations. A major issue is the quality of the closed circuit television (CCTV) footage used, particularly the frame rate which can vary from 25 frames per second to one frame every 4s. To date, no study has investigated the effect of frame rate on forensic gait analysis. A single subject was fitted with an ankle foot orthosis and recorded walking at 25 frames per second. 3D motion data were also collected, providing an absolute assessment of the gait characteristics. The CCTV footage was then edited to produce a set of eight additional pieces of footage, at various frame rates. Practitioners with knowledge of forensic gait analysis were recruited and instructed to record their observations regarding the characteristics of the subject's gait from the footage. They were sequentially sent web links to the nine pieces of footage, lowest frame rate first, and a simple observation recording form, over a period of 8 months. A sample-based Pearson product-moment correlation analysis of the results demonstrated a significant positive relationship between frame rate and scores (r=0.868, p=0.002). The results of this study show that frame rate affects the ability of experienced practitioners to identify characteristics of gait captured on CCTV footage. Every effort should therefore be made to ensure that CCTV footage likely to be used in criminal proceedings is captured at as high a frame rate as possible.
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Admin2 · Nov 24, 2013

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Other threads tagged with forensic podiatry and gait analysis

NewsBot · Nov 29, 2013

Variability and Similarity of Gait as Evaluated by Joint Angles: Implications for Forensic Gait Analysis
Sylvia X.M. Yang, Peter K. Larsen , Tine Alkjær , Erik B. Simonsen , Niels Lynnerup
Journal of Forensic Sciences; Early View

Closed-circuit television (CCTV) footage is used in criminal investigations to compare perpetrators with suspects. Usually, incomplete gait cycles are collected, making evidential gait analysis challenging. This study aimed to analyze the discriminatory power of joint angles throughout a gait cycle. Six sets from 12 men were collected. For each man, a variability range VR (mean ± 1SD) of a specific joint angle at a specific time point (a gait cycle was 100 time points) was calculated. In turn, each individual was compared with the 11 others, and whenever 1 of these 11 had a value within this individual's VR, it counted as positive. By adding the positives throughout the gait cycle, we created simple bar graphs; tall bars indicated a small discriminatory power, short bars indicated a larger one. The highest discriminatory power was at time points 60–80 in the gait cycle. We show how our data can assess gait data from an actual case.
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NewsBot · Oct 24, 2015

How useful is thematic analysis as an elicitation technique for analyzing video of human gait in forensic podiatry?
Browne, T., Curran, M. and Vernon, D.
Journal of Forensic Identification. 65(6), pp. 999-1012. 0895-173X.

The aim of this study was to evaluate how useful thematic analysis is in the elicitation of observations of gait from a video recording. This was undertaken by providing a video recording of human gait to “novice” and “expert” podiatry students. The observations were explored using the qualitative tool of thematic analysis. The exploration of human gait using this technique gave a rich abundance of information and demonstrated that a basic level of experience or knowledge is required to provide a simple description of human gait. With more expertise came a richer description of observation of human gait by the “expert” group compared to basic observations by the “novice” group. Thematic analysis allows the use of language and the depth of the information to be evaluated when observing human gait from a video recording.
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NewsBot · Oct 25, 2016

Evidence Evaluation of Gait Biometrics for Forensic Investigation
Imed Bouchrika
Multimedia Forensics and Security; 18 October 2016

Due to the unprecedented growth of security cameras and impossibility of manpower to supervise them, the integration of biometric technologies into surveillance systems would be a critical factor for the automation of security and forensic analysis. The use of biometrics for people identification is considered as a vital tool during forensic investigation. Forensic biometrics concerns the use of biometric technologies to primarily determine whether the identity of the perpetrator recorded during the crime scene can be identified or exonerated via a matching process against a list of suspects. The suitability of gait recognition for forensic analysis emerges from the fact that gait can be perceived at distance from the camera even with poor resolution. The strength of gait recognition is its non-invasiveness nature and hence does not require the subject to cooperate with the acquisition system. This makes gait identification ideal for situations where direct contact with the perpetrator is not possible.
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NewsBot · May 13, 2017

Expert studied how Exeter murder suspect walked after body was dumped, trial hears

NewsBot · Aug 9, 2022

A preliminary study investigating the overlay method in forensic podiatry for comparison of insole footprints
NicolasHowsamSarahReelJennyKilley
Science & Justice Volume 62, Issue 5, September 2022, Pages 494-505

Highlights
•Findings support advice that the overlay method should not be used in isolation.
•Agreement between two examiners’ comparisons showed high inter-rater consistency.
•One examiner showed poor agreement with two other examiners’ overlay comparisons.
•Whilst a lack of consistency was found, this is ameliorated at verification stage.
•Validity of method: Sensitivity: 77.8%; Specificity: 61.9%; FPR: 38.1%; FNR: 22.2%.
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The overlay method is used in forensic podiatry to compare the shape, position, and overall fit of features between questioned and reference footprint evidence. However, the scientific foundation underpinning its validity in the comparison of insole footprints is not fully understood because of a lack of published data defining its statistical operating parameters. A review of literature revealed its subjective nature with little scientific validation, limited only to face validity.

The aim of this study investigated strength of agreement between three expert footprint examiners’ overlay comparisons of ten reference insole footprints using Cohen’s weighted kappa (Kw). Validity of method was explored using measures of sensitivity, specificity, false positive rate (FPR), false negative rate (FNR), receiver operating characteristic (ROC) curve and area under the ROC curve (AUC). Results showed two examiners demonstrated high inter-rater consistency between their overlay comparisons (Kw: 0.981; 95 %CI: 0.943 to 1.020 for Rater1 v Rater2) whilst one examiner did not (Kw: 0.340; 95 %CI: 0.148 to 0.532 for Rater1 v Rater3; Kw: 0.310; 95 %CI: 0.100 to 0.519 for Rater2 v Rater3). Validity was investigated using a conclusion scale in a verbal expressions table to indicate support or rejection of compatibility of footprints between one questioned insole and ten reference insoles. Data analysis indicated validity as follows: Sensitivity: 77.8 %, Specificity: 61.9 %, FPR: 38.1 % and FNR: 22.2 %. ROC analysis corroborated this finding. AUC was calculated at 0.762 or 76.2 % indicating an ‘acceptable’ measure of overall accuracy of the overlay method for insole footprint comparison. Findings provide novel data supporting previous suggestions that the overlay method should not be used in isolation to compare insole footprints. Data also offers insight into the scientific foundation of this method, whilst highlighting its limitations and providing some implications and recommendations for forensic podiatry practice.
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