Crime Scene Investigator Network Newsletter

This blind study looked at the accuracy of 101 individuals and how well they could mark a set of 18 digital images of perfect ellipses.

Bloodstain pattern analysis relies heavily on the understanding and classification of bloodstains found at a crime scene. Analysts examine the size, shape, distribution and location of the bloodstains to try and determine things such as where an assailant may have been located or where a victim was struck. When considering impact stains, both the area of convergence and area of origin may be calculated to demonstrate where the impact took place in two -dimensional or three-dimensional space. Traditional methods of stringing and newer, digital methods often involve identifying and selecting bloodstains to include in an analysis. Part of this process requires that the analyst measures the width and length of each bloodstain and often, this can prove to be a difficult exercise since bloodstains are not always found in perfect form. Depending on factors such as environmental conditions and surface properties of a target surface, bloodstains may be malformed and require some knowledge to interpret the correct shape.

Marking errors resulting from incorrectly marked bloodstains (i.e. the marked shape of the ellipse does not agree with the known impact angle or true width to length ratio of the bloodstain) contribute to the overall errors in an area of origin or convergence analysis. Errors may be due to an analyst's subjective interpretation which may be complicated by malformed bloodstains and estimations of where to place an accurate ellipse around an individual bloodstain. One question which has not been addressed in ellipse marking studies is performance when bloodstains are near perfect and photographs are of high quality. Thus, this study addresses a best-case scenario for analysts when most problems associated with bloodstain ellipse marking have been removed.

There are two conditions that can exist for any bloodstain. Bloodstains may be circular (i.e. where a blood drop hits a surface at 90° and the length and width are equal), or they may be elliptical, with the length making up the major axis and the width, the minor axis (Figure 1). Passive blood drops such as those dripping from a bloody nose while a person is standing still, appear circular, while blood drops from a person moving (i.e. having any motion component parallel to the impacted surface) will exhibit a more elliptical shape (Figure 2).

The impact angle, which is directly related to the shape of the bloodstain, can be expressed by the equation:

where Θ is the impact angle of the bloodstain, w is the width of the bloodstain (minor axis, e.g. 5 mm) and l is the length of the bloodstain (major axis, e.g. 10 mm).

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Related Content

• Blood in Murder Investigation, by Sinisa Franjic
• Computers vs Strings - Two Cases in Point, by Daniel J. Rinehart
• Development of a Science Base and Open Source Software for Bloodstain Pattern Analysis, by Daniel Attinger. Ph.D.
• The Fluid Dynamics of Droplet Impact on Inclined Surfaces with Application to Forensic Blood Spatter Analysis, by G. Paul Neitzel and Marc Smith

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