Evidence Recognition, Collection and Preservation
What this domain tests
Questions test Locard's exchange principle, recognition of physical evidence, and selection of collection and packaging methods that preserve class and individual characteristics.
Core rules and concepts
- Evidence must be relevant, legally obtained, and documented before collection.
- Collect fragile and transient evidence first: DNA, blood, impressions, trace, then stable items.
- Individual characteristics (fingerprint, DNA, toolmark) narrow to one source; class characteristics (shoe size, paint layer) narrow to a group.
- Package biological evidence in paper and air-dried; use paper for damp items; use plastic only for dry non-biological items or protected sharps.
- Collect known and reference samples and substrate controls; label packaging with item number, collector, date, and location.
Worked example
A bloody shirt is draped over a chair. You photograph it in place, let it air-dry in a secure room, place it in a paper bag with a substrate control from an unstained area, seal, and label. Reasoning: moisture in plastic promotes mold and DNA degradation; the substrate control helps distinguish blood from dye or background.
Traps
Packaging wet blood in plastic; collecting a stain without photographing its position; mixing known and unknown samples; handling with bare hands; failing to collect controls; over-packaging a large item and losing trace.