Chapter 36 — 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 wildlife forensics in one sentence, and name the single most foundational task it performs.

  2. Define environmental forensics, and state the three questions it most often answers about a contaminant.

  3. † Define forensic engineering and failure analysis, and explain how failure analysis is the scientific method (Chapter 5) applied to wreckage.

  4. Define accident reconstruction, and name three kinds of physical evidence it reads to reconstruct a collision.

  5. Distinguish, in wildlife forensics, a species identification from an individual/population identification, and match each to the class-versus-individual distinction (Chapter 1).

  6. What is a DNA barcode as used in species identification, and how is the comparison-to-a-database logic the same as CODIS or AFIS?

  7. In fire-and-explosion engineering, distinguish cause arcing from victim arcing, and state which one indicates an electrical cause of the fire.

  8. What is an event data recorder (EDR), and why is it often the single strongest piece of evidence in an accident reconstruction?

B. Applied reasoning

  1. † A tusk's DNA is compared against a reference map of how elephant DNA varies across the continent, and the analyst reports a likely region of origin. Explain how this is the same population-genetics logic that underlies the random match probability (Chapter 7), and state the honest verb for the conclusion (is it a "coordinate" or an "association"?).

  2. An oil sheen on a harbor has weathered for five days. Explain why the analyst compares weathering-resistant biomarker ratios rather than the lighter, more volatile compounds, and what that allows.

  3. † A groundwater plume of a common solvent is "matched" by chemical fingerprinting to factory X. An unsampled factory Y, upgradient, used the same solvent. Explain how the attribution to X could be wrong, and name the two factors that most control the real strength of a chemical source attribution.

  4. A metal component's fracture surface shows progression ("beach") marks followed by a region of sudden rupture. What failure mode does this indicate, and what three things can an engineer often infer from such marks?

  5. A skid mark is 24 meters long on dry asphalt of known friction. Explain why the speed this yields is a minimum speed and a range, not a single speedometer reading — and connect the "minimum" logic to the entomologist's minimum postmortem interval (Chapter 13).

  6. Walk the failure-analysis fault tree (Figure 36.x in the chapter) for a collapsed load-bearing connection. List the four candidate-cause branches and, for each, name one piece of physical evidence you would test it against.

  7. Why is "we examined the electrical system and affirmatively determined it did not cause this fire" a far stronger statement than "we found no accidental cause, so it must be arson"? Name the fallacy the second statement commits (Chapter 22).

C. Evidence interpretation

  1. † Re-read Figure 36.1 ("Reading speed from the road"). List two things the skid-and-crush evidence can establish at honest strength, and two things it cannot establish from the physical marks alone.

  2. An engineer's report concludes: "Based on the progression marks on the fracture surface and confirmatory materials testing, the component failed by fatigue initiating at the keyway, consistent with the cyclic loads documented in service." Identify three ways this sentence is appropriately honest (where it ties the conclusion to affirmative evidence).

  3. A news article says a "chemical fingerprint" has "identified" the company responsible for a spill "beyond doubt." Rewrite the claim at honest strength, and name the specific overreach the word "fingerprint" smuggled in.

  4. A forensic engineer finds melted, beaded wiring near a fire's origin and a colleague concludes the fire was electrical. Using §36.5, explain why the melted wire alone does not support that conclusion, and what affirmative analysis is required instead.

D. Spot the overstatement / junk-science alert

  1. † An expert testifies: "The chemical fingerprint proves this spill came from the defendant's tank and no other source on Earth." Identify two distinct problems with this statement and rewrite it honestly.

  2. A wildlife analyst testifies that a degraded, carved ivory fragment is "definitely African elephant." The sequence recovered was partial and could not be distinguished from a closely related species not in the reference database. Name the overstatement and give the defensible version.

  3. A reconstructionist testifies that "the skid marks prove the defendant was driving recklessly and ran the red light." Separate the defensible physics claim from the unsupported narrative claim, and explain which is which.

  4. An engineering report concludes a product was defective because the engineer "could not think of any other reason it would fail." Name the fallacy (Chapter 22, in a hard hat) and state what the report needed instead.

E. Ethics and reasoning

  1. † A forensic engineer is retained by a manufacturer and told, before examining anything, that "obviously the user misused the product." Using the Cognitive-Bias Watch in §36.3 (and Chapter 31), explain the risk and the safeguards. Is a conclusion reached by testing only the client's preferred hypothesis worth more or less than one reached by testing all of them?

  2. Two qualified accident reconstructionists, retained by opposing parties, reach different speed ranges that nonetheless overlap. Is this necessarily a sign that one is dishonest? Explain by reference to the inputs (friction value, measurements, assumptions), drawing the parallel to the dueling entomologists of Chapter 13.

  3. You are asked to testify that a chemical "fingerprint" individualizes a pollution source the way DNA individualizes a person. Explain why you should decline, and what you can honestly say instead.

  4. Why is the completeness of the reference database (wildlife DNA) and the completeness of the candidate-source set (environmental attribution) each an ethical as well as a technical issue — i.e., why must an honest analyst disclose the gap rather than report the match as if the set were complete?

F. Synthesis and validity spectrum

  1. † Place these four claims on the NAS 2009 / PCAST 2016 validity spectrum (strong → weak), justifying each: (a) "this sequence is consistent with African elephant and excludes the substitute species tested" (§36.1); (b) "this compound is diesel fuel" (§36.2); (c) "this spill came from this specific tank" (§36.2); (d) "this fracture began at this specific flaw and propagated by fatigue" (§36.3).

  2. Explain how this chapter's finding in the cold case (electrical system ruled out as an accidental cause) complements the Chapter 22 incendiary finding from the opposite direction, and why both are needed for an honest staged-homicide reading.

  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, using at least two of the chapter's domains.

G. Cold-case extension

  1. † Cold Case. Using only what the forensic-engineering failure analysis establishes, write the entry you would add to the Mill Creek evidence log (Appendix I). State (a) the defensible finding at its true strength, (b) why it is an exclusion rather than an inclusion, (c) how it relates to (without duplicating) the Chapter 22 incendiary finding, and (d) why it adds no name to the file.

  2. Cold Case extension. A defense attorney argues: "The investigators just couldn't find an accidental cause, so they assumed arson — exactly the Willingham error." Explain how the forensic-engineering analysis in this chapter is the answer to that argument rather than an instance of it, and what specifically distinguishes the two.

  3. Cold Case, integrative. The engineering exclusion closes an accidental-cause alternative but names no suspect. List two other evidence types from earlier chapters that, combined with the engineering and arson findings, begin to connect a person to the fire — and state plainly why the engineering finding alone cannot make that leap.

H. Short writing

  1. In 150–200 words, explain to a juror why "the engineer proved why the building collapsed" is the wrong way to think about a failure analysis, and how to think about it instead (hypotheses, affirmative evidence, what the analysis can and cannot establish).

  2. † In 150–200 words, contrast the validity foundations of wildlife DNA species identification with those of environmental source attribution: what does each rest on, where is each strong, and where is each most easily overstated in court?

  3. In 150–200 words, defend the chapter's central claim — that the same evidentiary logic governs these domains as governs the homicide lab — using the class-versus-individual distinction (Chapter 1) and at least three of the chapter's five domains.