Chapter 19 — 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 trace evidence in one sentence, and explain why the chapter says it is defined "less by what it is than by how little of it there is and how it got there."

  2. State Locard's exchange principle (Chapter 3) in your own words, and explain why the chapter calls trace evidence "the physical face" of that principle.

  3. † Distinguish transfer from persistence. Then explain why a fiber that "transfers readily and persists for days tells a different story than one that transfers poorly and is gone in an hour."

  4. Distinguish primary from secondary transfer. Name the earlier chapter (and the kind of DNA) where you first met the secondary-transfer problem.

  5. Define hair morphology, and name the three layers of a hair shaft that an examiner reads.

  6. Define microscopic hair comparison, and state in one sentence the single thing the chapter says it can support and the single thing it cannot.

  7. Define fiber, and distinguish a natural from a manufactured/synthetic fiber with one example of each.

  8. Define the comparison microscope, and name two other disciplines in this book (besides hair and fibers) that depend on it.

  9. List the four trace-recovery methods named in the §19.1 "At the Bench" callout, in the chapter's rough order of preference, and state the trade-off every one of them makes.

  10. What is the single most reliable thing a microscopic hair examination can establish? (Hint: it has nothing to do with comparison to a suspect.)

B. Applied reasoning

  1. A hair recovered from a victim is described as "human, a head hair, naturally shed, dark brown, medium diameter, fragmented medulla." For each of those six descriptors, label it as a class characteristic or an individual characteristic, and state what follows for the strength of any "match."

  2. † An examiner wants to convert a hair "consistency" into a likelihood ratio (Chapter 9). Explain, step by step, why this is impossible for a visual hair comparison. What specific quantity is missing, and why was it never established?

  3. Explain why the chapter says microscopy's honest job in the modern lab is "to route hairs to DNA, not to render a comparison verdict of its own." What three things does the examiner determine about a hair that decide which DNA test (if any) it can receive?

  4. A rootless hair is recovered. Why can it not be typed with nuclear STRs (Chapter 7), and what kind of DNA (Chapter 8) is it sent for instead? State honestly what that second kind of DNA can and cannot do.

  5. Two blue acrylic fibers "look identical" to the naked eye. Describe the bench step from §19.4 that could nonetheless exclude them as having a common source, and explain why exclusion here is "decisive."

  6. † A suspect's jacket and a victim's body each yield fibers matching the other person's clothing — a two-way exchange of several different fiber types, one of them an unusual custom color. Explain why this association can be genuinely strong even though "fibers are class evidence" and "no fiber individualizes." Name the two factors that scale a fiber association's strength.

  7. Explain why a single cotton fiber linking a suspect to a scene is nearly worthless, while a single custom-dyed trilobal nylon fiber from a particular carpet can carry real weight. Tie your answer to the concept of "the value of the rare."

  8. The comparison microscope is described as "a superb tool for excluding and a treacherous one for confirming what you hoped to see." Explain the mechanism that makes the same instrument a strength in one direction and a trap in the other.

  9. Distinguish ordinary trace class evidence from a physical (jigsaw / fracture) match (Chapter 1). Why can a fracture match be near-individualizing when a hair "match" cannot? Give one example of each from the chapter.

  10. Microspectrophotometry "records the fiber's color as a full absorption spectrum rather than a subjective adjective." Explain why measuring color as a spectrum is more defensible than an examiner reporting "they're both navy blue," and state what a difference between two spectra establishes.

C. Evidence interpretation

  1. † Re-read Figure 19.1 ("A single hair under the comparison scope"). List the defensible observations the figure says the exam shows, then list what it says the exam does not show. Finally, state the figure's "honest reading" using the correct forensic verb, and explain why that is "the ceiling of the visual exam."

  2. An examiner's report on a hair reads: "The questioned hair exhibits no significant differences from the known hair and cannot be excluded as having originated from the same source; this is common-type hair shared by an unknown but large number of individuals, and no statistical weight attaches to the association." Identify three separate things this sentence does well (i.e., where it is appropriately honest).

  3. The cold-case Case File reports a "small number of synthetic fibers" on the victim's shirt that are "consistent with" a jacket later associated with Roy Keller. Write the single sentence an honest fiber examiner could say on the stand about that evidence, and the single sentence that would be an overstatement.

  4. The cold-case hair is rootless and goes for mtDNA, which returns a result recorded as "a non-exclusion of limited weight, not an identification." Explain why mtDNA "cannot individualize either" — what biological fact about mitochondrial DNA (Chapter 8) limits it — and why a non-exclusion of limited weight is still recorded honestly rather than discarded.

  5. An examiner testifies that two hairs, viewed side by side in a comparison microscope, are "a microscopic match." Translate that phrase into the strongest claim the visual science actually supports, and name the instrument-specific bias (from §19.5) that made the "match" feel objective.

D. Spot the overstatement / junk-science alert

  1. A 1990s transcript reads: "The hair recovered from the victim matched the defendant's hair; in my experience the chances of it coming from someone else are extremely small." Identify the three distinct overstatements from §19.3 embedded in that sentence, and rewrite the testimony at its true strength.

  2. † A prosecutor's slide reads: "Microscopic analysis places the defendant's hair on the victim's body." Name the specific overstatement (what is being claimed that hair comparison cannot support), and give the defensible version. Which earlier chapter's term names the leap from "this type of hair" to "this person's hair"?

  3. An examiner assigns "a one-in-a-thousand chance" that a questioned hair came from someone other than the defendant. The chapter calls a statistic like this, however sincerely offered, "a fabrication." Explain why — what does the number lack that a DNA random match probability (Chapter 7) has?

  4. A defense expert claims, "The 2015 FBI review proves hair evidence is worthless and that every defendant convicted with hair testimony is innocent." Using the "two honest cautions" at the end of §19.3, identify the two separate overclaims in that statement and correct each.

  5. A television examiner glances at a hair, declares "that's our guy," and a database produces a photo. Using §19.2–19.3, give two reasons this is backward from how honest hair examination actually works.

E. Ethics and reasoning

  1. † The 2015 FBI hair review found systematic overstatement "at the most prestigious forensic laboratory in the country," not at a disreputable one. Using the Cognitive-Bias Watch in §19.3 (and previewing Chapter 31), explain why the prestige of the lab makes the scandal more instructive about cognitive bias, not less. Name the missing safeguard that would have caught the drift.

  2. The §19.1 Cognitive-Bias Watch says "a 'match' that your own laboratory manufactured is worse than no evidence at all." Explain the reasoning, and describe the concrete collection-and-examination practices the callout prescribes to prevent it.

  3. You are asked to testify that a hair "individualizes" a defendant the way DNA individualizes a person. Explain why you should decline, and state precisely what you can honestly say about a hair from a visual examination instead.

  4. An examiner privately means only "consistent with" but, on the stand, uses the word "match" because the prosecutor and the jury understand it more easily. Explain why §19.3 treats this as a forensic failure even when the examiner is not lying, and connect it to Theme 4 (the CSI effect cutting both ways).

  5. The review "faulted the testimony, not necessarily the verdicts." Construct the ethical argument for why overstating weak evidence is wrong even when the defendant turns out to be guilty. Why is "he did it anyway" not a defense of the overstatement?

F. Synthesis and validity spectrum

  1. † Place these four methods, justifying each, on the NAS 2009 / PCAST 2016 validity spectrum (strong → discredited): single-source nuclear DNA (Chapter 7); honest fiber comparison (this chapter); microscopic hair comparison for association with a person (this chapter); bite-mark comparison (Chapter 16). Explain in one sentence why fiber and hair sit so far apart even though "both are class evidence under the same microscope."

  2. Explain why the chapter calls the comparison microscope "the perfect emblem of the whole chapter." What is the single sentence (paraphrase it) that captures why the tool was never the problem?

  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.

  4. The chapter argues that "a method with no measured error rate, operated inside a system that knows the answer it wants, will drift toward that answer and call the drift science." Show how this single sentence connects the hair scandal (§19.3) to the fingerprint chapter's Brandon Mayfield case (Chapter 14) and to the bias chapter (Chapter 31).

G. Cold-case extension

  1. † Cold Case. Using only what the trace evidence in this chapter's Case File establishes (the fibers and the hair), write the two entries you would add to the Mill Creek evidence log (Appendix I). For each item state (a) the defensible inference at its true strength, (b) the honest verb, (c) at least two things the item specifically does not establish, and (d) why you decline to attach Roy Keller's name to it as a conclusion at this stage.

  2. Cold Case extension. The Case File notes what would strengthen the trace. Propose, for the fibers and for the hair separately, one concrete additional finding that would honestly increase the weight of each association, and explain what each would add and why.

  3. Cold Case, integrative. The chapter says, "in 1995, this hair might have 'matched' Roy Keller, and that sentence would have been a lie. In our file, it does not." Explain what the post-2015 examiner did differently with the same hair, and name the two later chapters whose evidence (not the hair) would actually be needed to tie a person to the cabin.

H. Short writing

  1. In 150–200 words, explain to a juror why a hair on a victim's body "sounds like proof" but, by visual examination alone, usually is not — and what single thing (from Chapters 7–8) can change that.

  2. † In 150–200 words, contrast the validity foundations of honest fiber comparison with those of microscopic hair comparison: what does each rest on, where is each strong, and why did one discipline learn to state its conclusions at their true strength while the other, for decades, did not?