Chapter 16 — Key Takeaways

A scannable one-page card. For the full argument and the worked examples, see index.md.

The core claims

• One physics, one logic. All impression evidence rests on the same fact: a harder surface presses, scrapes, or cuts into a softer one, and the softer one keeps a record of the shape that touched it. Footwear, tire, tool, and bite marks are all governed by the class-vs-individual logic of Chapter 1 and are a legible form of Locard's exchange principle (Chapter 3).
• The hierarchy of conclusions runs: exclusion → class association → individual identification → inconclusive. Exclusion (a genuine, unexplained difference) is the cleanest, most defensible call. Agreement only associates; how strongly depends on whether the agreeing features are class or individual. The chapter's whole validity argument is about which methods can honestly reach which rung.
• Recovery comes before comparison — and you can lose the evidence forever. For footwear and tires: photograph first (lens parallel, scale in plane, oblique light to throw 3-D detail into relief), then cast three-dimensional impressions in dental stone (or lift two-dimensional ones). A cast is a positive replica of the source from the soil's negative. An impression you walk past, or cast too thin, is gone.
• Footwear and tire class evidence is genuinely strong. Tread design, size, and dimensions are manufacturing information: they can yield a make/model and narrow an investigation honestly. A tire places a vehicle at a scene — not a driver, and not a time.
• Footwear/tool individual identification is a judgment, not a calculation. It shares the firearms/toolmark predicament (Chapter 15): plausible in principle, durable in its features, but under-validated as a quantified claim, dependent on examiner judgment, and threatened by the subclass trap (a production-run match masquerading as an individual one). Defensible as "consistent with"; contested as "to the exclusion of all others" (PCAST 2016).
• Bite-mark analysis is the field's most thoroughly discredited discipline. It claims an examiner can identify who made a patterned bite, especially on skin. Its premises — that dentition is unique, that skin records that uniqueness, that examiners can reliably link a mark to a biter — were asserted, never validated. Skin (elastic, mobile, healing, distorting) is the worst imaginable substrate. When tested, examiners disagreed with one another and the ground truth — sometimes on whether a mark was even a human bite. It helped convict Ray Krone and Roy Brown, both later cleared by DNA.
• A discipline is not valid or invalid as a whole — a specific claim is. The same odontologist who cannot reliably tell whose teeth made a bruise can reliably identify a body from its dental records (Chapter 17). Always ask which claim is being made, and whether anyone has measured how often it fails.

The method-validity verdict (NAS 2009 / PCAST 2016)

Where they sit: footwear/tire/toolmark class evidence is in the defensible middle (grounded in manufacturing fact); their individual-source claims share the firearms band of the spectrum (contested, under-validated); bite-mark identification sits at the far discredited end, opposite DNA — the widest the validity spectrum gets, all inside one chapter.

What you can honestly say on the stand

• Footwear: "This impression is consistent with the class characteristics — the tread design and size — of the suspect's shoe, and of all other shoes of that design and size." (Exclusion, where warranted, is the cleanest call. Individual identification is an opinion about specificity — state how many features, how distinctive, and against what error rate.)
• Tire: "The impression is consistent with a tire of this tread design and width; such a tire could have made it. This places a vehicle, not a driver, at the scene, and says nothing about when."
• Toolmark: "The marks are consistent with a flat pry bar approximately 19 mm wide; the suspect's crowbar could have made them — which supports forced entry." (Class-level. Individual-source toolmark ID is in the contested band, PCAST 2016.)
• Bite mark: at most "this injury is consistent with a human bite," and possibly "the suspect cannot be excluded" — where any of it is admitted at all.
• What you must NOT say: "it's the defendant's shoe/tire/tool, to the exclusion of all others" presented as a calculation; "these teeth and no others made this mark"; or any conversion of a class association into an individual identification.

Key terms (one line each)

• Toolmark — any impression, scratch, gouge, cut, or abrasion a harder tool leaves on a softer surface; analyzed as class (impressed) and, contestably, individual (striated) evidence.
• Impression evidence — physical evidence in which one object's shape is recorded on another's surface (footwear, tire, tool, bite), governed by class-vs-individual logic.
• Footwear impression — the recorded outline and tread detail of a shoe's underside; class (tread, size, wear) and acquired (cuts, gouges, lodged debris) characteristics.
• Tire impression — the recorded tread detail of a vehicle's tire; class characteristics (design, width, dimensions, noise treatment) can yield make/model and place a vehicle (not a driver) at a scene.
• Cast — a 3-D reproduction of an impression, made by pouring a casting medium (typically dental stone) into a 3-D impression to create a positive replica for comparison.
• Bite-mark analysis (debunked) — the discredited claim that an examiner can identify the person who made a patterned bite injury, especially on skin; unvalidated, fails error-rate testing, multiple wrongful convictions.

The cold-case line

The cabin door's pry marks are a class match to a flat pry bar → forced entry supported (a significant crack in the "accidental fire" story; a dead man does not force his own door from outside). A partial shoe impression in the soil is logged as class evidence (tread design + approximate size), pending comparison. And there are no bite marks — the debunked method taught by its absence; the most valuable thing it contributes here is the discipline of not using it. Status after Ch. 16: forced entry supported; a footwear impression logged as class evidence; no person yet placed at the door.

The themes this chapter advanced

• Exclusion over proof — the hierarchy of conclusions puts exclusion first; class evidence constrains without naming; Brown's case is the tragedy of an exclusion ignored.
• The validity spectrum — the same family of evidence holds defensible class work and the field's most discredited discipline; the single separating question is the PCAST one: is there a validated method, with a known error rate, for the specific claim being made?
• (Also carried: cognitive bias — the examiner shown a suspect and asked to confirm a match, §16.2 and §16.5; and the CSI effect cutting both ways — Krone and Brown juries over-trusting a confident bite-mark exhibit, §16.6.)