Chapter 16 — Exercises

Work these without looking back at the chapter first; then check yourself. Items marked † have full worked solutions in the answers appendix. There are no answers in this file. Mix of recall, applied reasoning, evidence interpretation, "spot the overstatement," ethics, and a cold-case extension.

A. Recall and definitions

  1. Define impression evidence in one sentence, and state the single piece of physics that all impression evidence rests on.

  2. Define toolmark, and distinguish the two broad types — impressed (compression) marks and striated marks — by saying which carries mostly class information and which is, in theory, where individual identification might live.

  3. † Define a cast. Explain, in your own words, why the soil holds a negative of a shoe's sole while the cast is a positive, and why that distinction matters for comparison.

  4. Define footwear impression and tire impression, and name two class characteristics and one acquired (individual) characteristic for each.

  5. State the four-rung hierarchy of conclusions an honest impression examiner works within (from §16.1), in order, and give a one-line gloss of each.

  6. What does it mean to say that firearms identification is a special case of toolmark identification (Chapter 15)? Name one way a fired bullet's marks are more constrained than a general toolmark.

  7. Define bite-mark analysis as the discipline is described in this chapter, and state in one phrase why it is placed at the discredited end of the validity spectrum.

  8. Name the two further premises (beyond "the human dentition is unique") that bite-mark identification requires, and say which one skin, as a recording medium, defeats.

B. Applied reasoning

  1. The recovery order for a three-dimensional footwear impression is fixed: photograph first, then cast. Give two distinct reasons the photograph is taken first and is never skipped, even when a cast will also be made.

  2. † An impression in soil will be photographed at the scene. Describe how the examination-quality photograph should be set up — camera orientation, the scale, and the lighting — and explain what the oblique light specifically accomplishes that flat light does not.

  3. A vehicle leaves a tire impression running several feet along a muddy track. Explain why the examiner casts it in sections, and what must be recorded about those sections so the lab can use them.

  4. † A toolmark examiner finds fine parallel striations in a questioned mark that line up well with test marks from a suspect's chisel. Before concluding "individual identification," what is the one manufacturing-related possibility (from Chapter 1) that must be ruled out, and why can it masquerade as an individual match?

  5. Why does a pry bar produce marks that vary far more from use to use than the marks a rifled barrel leaves on successive bullets? Name two variables the examiner must reproduce when making test marks.

  6. A burglary scene shows a door splintered around the lock and a clear partial shoe impression in the flower bed beneath the window. State, for each item separately, what kind of investigative question it answers and the honest verb for the strongest defensible conclusion.

  7. Sequence these four impression-comparison conclusions from most defensible to least defensible, and justify the ordering: (a) "consistent with the class characteristics of the suspect's shoe"; (b) "could not have been made by the suspect's shoe"; (c) "made by the suspect's shoe, to the exclusion of all others"; (d) "the impression is too smeared to evaluate."

C. Evidence interpretation

  1. † Re-read Figure 16.1 (recovering a three-dimensional footwear impression). Explain in your own words why the dental-stone cast is a positive replica you can lay beside a suspect's shoe, and name the one handling error the figure's caption warns can "take the only record of the impression with it."

  2. Re-read Figure 16.2 ("Two methods, one defendant, opposite answers," after Ray Krone). The block says the later DNA result "does not retroactively make the bite-mark method 'almost right.'" Explain what that sentence is guarding against — what wrong lesson someone might draw — and state the correct lesson the figure teaches.

  3. An examiner's report on a forced door reads: "The marks in the doorframe are consistent with a flat pry bar approximately 19 mm wide; the suspect's crowbar could have made them." Identify three things this sentence does well (where it is appropriately honest about its own strength).

  4. A bite-mark report on a patterned bruise concludes that the suspect "cannot be excluded as the source of the bite." Even this restrained wording carries a hidden assumption that the chapter says is itself unvalidated. Name that prior assumption (what the examiner must first claim before "cannot be excluded" means anything), and explain why it is shaky.

D. Spot the overstatement / junk-science alert

  1. † A prosecutor's slide reads: "Toolmark analysis proves the defendant's crowbar, and only that crowbar, forced this door." Identify two distinct problems with this statement (one about what toolmarks can establish, one about whom a toolmark implicates) and rewrite it honestly.

  2. An odontologist testifies: "The bite mark on the victim matches the defendant's teeth; these teeth and no others made this wound." Name the specific overstatement, state what (if anything) a bite-mark examiner could honestly say instead under current standards, and explain why even that is contested.

  3. A television examiner glances at a single tire track and a suspect's car in the driveway and declares, "That's the car." Using §16.3, give two reasons this is backward from how tire-impression comparison actually works, and name what a tire impression places at a scene (and what it does not).

  4. A footwear report computes a confident "identification" from a partial impression but never states how many corresponding acquired features were found, how distinctive they were, or against what error rate. List the missing elements that should make a reader distrust the conclusion.

E. Ethics and reasoning

  1. † A toolmark examiner is handed the suspect's crowbar by the detective with the words "this is the one that did it — confirm it for me." Using the Cognitive-Bias Watch in §16.2 and previewing Chapter 31, explain the contextual-bias risk and describe the safeguard. Is a comparison reached before the examiner knows which tool the detective favors worth more or less than one reached knowing the answer — even if they agree?

  2. The Roy Brown case (§16.6) involved a bite mark that, by one account, displayed more upper teeth than Brown possessed. Explain why, in any disciplined comparison, an unexplained difference like that should drive toward exclusion — and what it means that the trial testimony "minimized rather than treated as the exclusion it arguably was."

  3. You are asked to testify that a footwear "identification" is as certain as a single-source DNA match. Explain why you should decline, drawing the distinction between a conclusion that is a calculation and one that is a judgment about specificity.

  4. The American Board of Forensic Odontology developed guidelines and a scoring vocabulary for bite-mark comparison. Explain why a professional board, credentials, and standardized forms do not, by themselves, supply the thing bite-mark analysis actually lacked — and name what that missing thing is.

F. Synthesis and validity spectrum

  1. † Place these four methods on the NAS 2009 / PCAST 2016 validity spectrum (strong → discredited), justifying each: single-source nuclear DNA (Chapter 7); footwear class evidence (this chapter); individual-source toolmark "identification" (this chapter); bite-mark identification (this chapter).

  2. Explain how this chapter is "the validity spectrum compressed into a single chapter" — how the same family of evidence (impressions and marks) contains both genuinely defensible work and the field's most discredited discipline. Name the single question (from §16.1 and the NAS/PCAST framework) that best separates the two ends.

  3. In one paragraph, explain how this chapter advances at least two of the book's four themes (exclusion over proof; the validity spectrum; cognitive bias; the CSI effect cutting both ways). Name which themes and how the Krone and Brown cases carry them.

G. Cold-case extension

  1. † Cold Case. Using only what the pry-mark evidence on the Mill Creek cabin door establishes, write the entry you would add to the evidence log (Appendix I). State (a) the defensible inference at its true strength, (b) the honest verb, (c) at least three things this evidence specifically does not establish, and (d) why "forced entry supported" still does not place any named person at the door.

  2. Cold Case extension. The cabin door's toolmarks support forced entry, which strains the original "accidental fire" story. Explain, in two or three sentences, why "a man dead in a fire does not, by himself, force his own door from outside" is a legitimate inference — and what kind of evidence (from earlier or later chapters) would be needed to move from "someone forced the door" to "this person forced the door."

  3. Cold Case, the absence. The Mill Creek file contains no bite-mark evidence, and the chapter calls that absence "itself this chapter's lesson." Explain what it would have cost the case if an over-eager examiner had found a patterned injury and "matched" it to a person of interest — and why, given Krone and Brown, "not using bite marks" is the disciplined choice rather than a missed opportunity.

H. Short writing

  1. In 150–200 words, explain to a juror the difference between a class match and an individual identification for a shoe impression — why "this model and size of boot, with this wear pattern" is solid, useful evidence, and why "it is the defendant's shoe and no other" asks the impression to do something it usually cannot.

  2. † In 150–200 words, contrast the validity foundations of footwear/tire class evidence with those of bite-mark identification: what does each rest on, where is the first genuinely strong, and what exactly is missing from the second that the 2009 NAS and 2016 PCAST reports identified as decisive?