Bloodstain Pattern Analysis
The Bloodstain Pattern Analysis domain tests classification, droplet physics, documentation, and the limits of reconstruction. Questions use SWGSTAIN/OSAC terminology and ask you to separate what a stain shows from what an analyst infers.
Core pattern rules
- Impact spatter is described by droplet size: low >4 mm, medium 1–4 mm, high <1 mm mist. Size is descriptive; it does not by itself prove a weapon or velocity.
- Cast-off is a linear trail from a bloodied object in motion; droplet size often decreases along the trail. A trail is not a blow count.
- Transfer is contact; flow and pool are gravity-driven; a void is an absence where an object blocked deposition.
- Directionality comes from the pointed tail, scallops, and satellite droplets, which point along travel away from the origin.
Physics and reconstruction
- Angle of impact: sin θ = minor axis ÷ major axis (width ÷ length of the ellipse).
- Area of convergence is a two-dimensional intersection of lines through stain long axes. Area of origin adds height: height = horizontal distance × tan θ.
- Typical drop: ~0.05 mL, ~4.6 mm diameter on a nonporous target; terminal velocity is roughly 7–8 m/s.
Worked example
An elliptical stain is 24 mm long and 8 mm wide. sin θ = 8/24 = 0.333, so θ ≈ 19.5°. Draw the line through the long axis back to the convergence point. If that point is 50 cm horizontally from the stain, height ≈ 50 × tan 19.5° ≈ 17.7 cm above the target surface. The tail points away from the origin, fixing travel direction.
Traps
Classifying by size alone ignores substrate and secondary spatter. Treating convergence as origin is a standard error. Counting cast-off trails as blows overstates the reconstruction. Mixing units in tangent calculations, photographing without a scale, and letting context drive pattern calls also cost marks; document first, interpret second.