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An eye tracking study of bloodstain pattern analysts during pattern classification

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Abstract

Bloodstain pattern analysis (BPA) is the forensic discipline concerned with the classification and interpretation of bloodstains and bloodstain patterns at the crime scene. At present, it is unclear exactly which stain or pattern properties and their associated values are most relevant to analysts when classifying a bloodstain pattern. Eye tracking technology has been widely used to investigate human perception and cognition. Its application to forensics, however, is limited. This is the first study to use eye tracking as a tool for gaining access to the mindset of the bloodstain pattern expert. An eye tracking method was used to follow the gaze of 24 bloodstain pattern analysts during an assigned task of classifying a laboratory-generated test bloodstain pattern. With the aid of an automated image-processing methodology, the properties of selected features of the pattern were quantified leading to the delineation of areas of interest (AOIs). Eye tracking data were collected for each AOI and combined with verbal statements made by analysts after the classification task to determine the critical range of values for relevant diagnostic features. Eye-tracking data indicated that there were four main regions of the pattern that analysts were most interested in. Within each region, individual elements or groups of elements that exhibited features associated with directionality, size, colour and shape appeared to capture the most interest of analysts during the classification task. The study showed that the eye movements of trained bloodstain pattern experts and their verbal descriptions of a pattern were well correlated.

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Notes

  1. Bloodstain patterns that result from an object striking liquid blood [22].

  2. Accuracy is the difference between the true and measured gaze direction [25].

  3. Precision is how consistent calculated gaze points are, when the true gaze direction is constant [25].

  4. The smallest rectangle containing an element [3].

  5. K-nearest-neighbour search method uses the Euclidean distance metric to find the nearest neighbour in X for each point in Y. Therefore, distances between each observation in Y and the corresponding closest observation in X can be retrieved [29].

  6. A bloodstain resulting from a blood drop dispersed through the air due to an external force applied to a source of liquid blood [22].

  7. A bloodstain pattern resulting from blood forced by airflow out of the nose, mouth, or a wound [22].

  8. Each fixation that is made by a participant adds a value to the colour map that is proportional to its duration [30].

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Arthur, R.M., Hoogenboom, J., Green, R.D. et al. An eye tracking study of bloodstain pattern analysts during pattern classification. Int J Legal Med 132, 875–885 (2018). https://doi.org/10.1007/s00414-017-1711-6

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