Unit 03 · Trace Evidence
Trace evidence is built on a simple idea: every contact leaves a trace. This unit covers Locard's exchange principle and the small transfers it predicts — hairs, fibers, glass, and soil — and how each is compared under the microscope. The through-line is the difference between class and individual characteristics: most trace evidence can show that two samples are consistent with a common source, but rarely that they came from one source and no other. Mastery means you can compare carefully and say exactly what your comparison does and does not establish.
| Criterion | Not yet | Approaching | Mastered |
|---|---|---|---|
| Locard's exchange principle | Cannot state why contact leaves transferable evidence. | States the principle but cannot predict what would transfer at a scene. | Uses Locard's principle to predict likely transfers and guide what trace evidence to collect. |
| Hair comparison & its limits | Treats a hair match as proof of identity. | Compares hair features but overstates what microscopy alone can conclude. | Compares hair by microscopy and states its limit — hair is class evidence, not individualizing without DNA. |
| Fiber comparison | Calls two fibers the same without examining them. | Notes color or type but ignores cross-section or dye. | Compares fibers by type, color, cross-section, and dye, and reports them as consistent, not identical. |
| Glass & soil analysis | Cannot describe a physical property that distinguishes samples. | Measures one property but draws a conclusion it cannot support. | Uses refractive index, density, and layering to compare glass and soil as class evidence. |
| Comparison microscopy & documentation | Uses the microscope without a side-by-side comparison or notes. | Compares samples but documents the work inconsistently. | Runs a controlled side-by-side comparison and documents technique, observations, and limits. |
| Integration (cross-domain) | Treats trace evidence as isolated facts; makes no cross-domain connection. | Names a link from trace evidence to history, reading, or writing but cannot defend why it matters. | Connects trace evidence to its anchor across History · Reading · Writing (plus chosen electives) and defends why the connection matters. |
“The questioned fiber matches the sweater’s fiber in type, color, cross-section, and dye, so I can report them as consistent with a common source. That’s class evidence — it narrows the field, but it doesn’t say this fiber and no other, and I won’t call it a match.”
“The hairs look the same under the scope, so they’re definitely from the same person — that proves she was there.”
You demonstrate this unit through a comparison-microscopy investigation — comparing questioned and known hairs, fibers, glass, or soil side by side and documenting what you see aloud — not a multiple-choice test. A criterion counts as mastered only when you can run the comparison and state honestly whether it shows class consistency or something stronger. Mastery is demonstrated, not awarded.
A 5-page clipboard packet — unit overview, key terms, the mastery rubric, anchor examples, and a score sheet you can print and grade against.