Bloodstain Pattern Analysis
Bloodstain Pattern Analysis
This domain tests classification of stains by the event that produced them, the documentation that supports the classification, and the geometry used to place an impact in three dimensions. Expect questions that describe a pattern in words and ask for mechanism, directionality, or a calculated angle.
Pattern families
SWGSTAIN-style terminology separates spatter - impact, cast-off, arterial, expiration - from non-spatter patterns such as transfer, flow, pool, saturation, and void. Passive drops fall under gravity and are circular on a horizontal surface; impact spatter is driven by force and produces a radiating field of small stains; cast-off comes from a bloody object in motion and often appears as a linear trail; arterial patterns show a gush-and-spurt rhythm from a breached vessel; expiration produces fine, often clustered droplets from the nose or mouth.
Direction and origin
A stain's tail points in the direction of travel. For an elliptical stain, sin θ = width ÷ length, where width is the minor axis and length the major axis. The point of convergence is found by drawing lines through the long axes of related stains on a 2D surface; the area of origin is then estimated by stringing or trigonometry from the convergence point and the impact angles.
Worked example
A stain measures 18 mm long and 6 mm wide. sin θ = 6 ÷ 18 = 0.333, so θ ≈ 19.5°. The stain is therefore shallow - the drop struck at a glancing angle - and its tail indicates travel toward the narrow end. Reporting "about 20°" with the formula shown earns the method mark; reporting only the number does not.
Traps
Measuring the major axis as width inverts the calculation; ignoring substrate texture and absorption; classifying a single stain as a pattern; claiming an area of origin from too few or too parallel stains; and treating satellite spatter as a separate event.