Bloodstain Pattern Analysis
The Bloodstain Pattern domain tests whether you can describe a pattern physically, name the mechanism that produced it, and defend the limits of what a stain proves. Expect terminology tied to SWGSTAIN/IABPA vocabulary, angle-of-impact and area-of-origin maths, documentation standards, and report wording.
Classification
- Spatter patterns form when a liquid is dispersed into droplets; non-spatter patterns include drip, flow, pool, saturation, transfer and voids.
- Impact, cast-off (linear, wave, spine), expiration, arterial and back spatter each carry distinguishing features: cast-off lines are linear and often parallel; expiration commonly contains bubbles or mucus; a void suggests an object intercepted the flight path.
- Legacy velocity bands (low >4 mm, medium 1–4 mm, high <1 mm) describe drop size, not speed, and never identify a weapon by themselves.
Physics
Angle of impact: sin θ = minor axis ÷ major axis. Direction of travel is read from the tail, the scalloped edge and the position of satellite droplets relative to the parent stain.
Worked example. A stain measures 12 mm long by 6 mm wide. sin θ = 6/12 = 0.50, so θ = 30°. If the point of convergence lies 40 cm away across the surface, origin height h = d × tan θ = 40 × tan 30° ≈ 23 cm above the surface. Two stains give a rough three-dimensional check; ten or more support a defensible origin area. Report a region, never a point.
Traps that cost marks
- Treating “high velocity” as proof of a firearm discharge.
- Quoting a drop height or weapon type from one stain.
- Confusing the direction a droplet travelled with the direction of the force that dispersed it.
- Reading overlaps as a timeline; sequencing requires scene context and other evidence.
- Missing transfer patterns on fabric or footwear that mimic impact spatter.