Quantitative Analysis of High Velocity Bloodstain Patterns
William Ristenpart, Fred Tulleners, Sonya Siu, Jennifer Saifi, Faye Springer
ABSTRACT
The goal of this study is to establish statistically significant classifications of blood spatter patterns resulting from the interactions between a weapon, suspect and victim. Specifically, a “medium velocity” spatter pattern is usually attributed to blunt force injury, while a “high velocity” pattern is typically attributed to a gunshot wound. The differentiation between these classifications, however, has been qualitative and controversial. There are neither supporting statistical data nor are there objective criteria as to what constitutes “consistency” or the associated error rate. In this study, high speed video (at >10,000 frames per second) was used to visualize simulated bloodshedding events. The impact velocity of various blunt instruments, including a bat, crowbar, and hammer, onto blood soaked sponges was varied systematically. Analogous experiments were also performed with different caliber bullets fired with systematically varied distances to the target surface. In each case, the spatter drop size distribution and morphology was digitized and quantified using a series of rigorous metrics, thereby developing a large statistical “library” of spatter patterns. Photographs of the patterns were then assessed by trained analysts in a double-blind fashion, with the goal of providing quantitative error rates and testing objective criteria for the classification of medium and high velocity bloodstain patterns. We obtained two key findings. First, we demonstrate that quantitative metrics involving the spatially-dependent size distribution of droplets within a spatter pattern could serve as an objective means of differentiating gunshot and blunt instrument spatter patterns. Second, our double blind investigation revealed that human assessments yielded low error rates for gunshot spatter patterns (0.2%), but very high error rates for blunt instrument spatter patterns (37%). Our findings strongly suggest that (i) great caution should be exercised when identifying a pattern as resulting from a gunshot or blunt instrument impact in the absence of secondary indicia, and (ii) that further effort should be put toward development and refinement of quantitative image analysis procedures based on droplet spatial distributions.