Chapter 14 — 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 friction ridge, and state the two biological facts (one about time, one about variation) that make friction ridge skin useful for answering "who touched this?"

  2. Distinguish a latent print, a patent print, and a plastic print in one sentence each, and give one example surface or scenario for each.

  3. † Define minutiae (level 2 detail), name three principal types, and explain why the configuration of many minutiae — not the count alone — is what carries identification value.

  4. Name the four steps of ACE-V in order, and state in a phrase what happens at each.

  5. What does an AFIS actually return at the end of a search? State the single word that best describes the output, and the single word that describes what it does not produce.

  6. Define level 1, level 2, and level 3 detail, and state for each whether it can exclude, identify, or only narrow — and why.

  7. Name the three broad pattern families (loop, whorl, arch) and give the one-line distinguishing feature of each. Which is the rarest?

  8. What is the uniqueness claim about fingerprints, and why does the chapter insist it is a claim about nature, not about examiners?

B. Applied reasoning

  1. † An examiner says, "Both prints are loops, so this is a match." Identify the single word doing dishonest work and explain, using the level-1/level-2 distinction and Chapter 1's class-vs-individual logic, exactly what is wrong.

  2. A latent is developed on a sheet of printer paper recovered from a desk, and a second latent is developed on the chrome handle of a refrigerator. Name an appropriate development method for each, name what each method targets in the residue, and state why you would not swap the two methods.

  3. Explain, step by step, why the order in which you process an item for prints versus touch DNA (Chapter 8) can decide whether you keep or lose evidence. Name one development method that is relatively DNA-friendly and the general class of methods that is not.

  4. † Why might eight matching minutiae sometimes constitute a stronger identification than sixteen? Frame your answer in terms of the quality, clarity, and rarity of the configuration, and explain what was wrong with the old fixed point-counting standard.

  5. An AFIS search of a partial latent returns a ranked candidate list whose number-one record scores far higher than the rest. The true source of the print has no record in any database. Explain what the number-one record is and is not, and why "the list always has a number one" is the dangerous part.

  6. Describe the difference between verification as commonly practiced and blind verification. Why does the chapter call ordinary verification the step where "the method's greatest weakness hides"?

  7. Level 3 detail (pore positions, ridge-edge shape) is described as "genuinely useful in high-quality comparisons and genuinely treacherous in poor ones." Explain the mechanism that makes the same feature helpful on a clean print and misleading on a smeared one.

C. Evidence interpretation

  1. † Re-read Figure 14.1 ("The match that wasn't"). List the five failure modes the chapter identifies in the Mayfield comparison, and for each, state in a phrase the safeguard that would have addressed it.

  2. An examiner's report reads: "I conducted an ACE-V comparison and found agreement in pattern and in 12 minutiae in corresponding positions, with no unexplained differences; in my opinion the latent and the exemplar originated from the same source." Identify three separate things this sentence does well (where it is appropriately disciplined).

  3. A latent of limited quality shows a pattern consistent with a loop and a handful of possible minutiae, several ambiguous. The named suspect's corresponding finger is also a loop. Explain why "consistent with a loop" does not place the suspect's hand on the object, and name the honest evaluation outcome an examiner should record.

  4. Two qualified examiners, working the same difficult latent in good faith, reach different conclusions — one "identification," the other "inconclusive." Is this necessarily evidence that one is incompetent or dishonest? Explain using §14.4's point about the absence of an objective threshold, and contrast it with what you would expect from two analysts reading a clean single-source DNA profile (Chapter 7).

D. Spot the overstatement / junk-science alert

  1. † A prosecutor's slide reads: "Fingerprint identification has a zero error rate, and this print is the defendant's to the exclusion of every other person on Earth." Identify two distinct scientifically indefensible claims in that sentence and rewrite the testimony honestly.

  2. An expert testifies that because "fingerprints are unique, fingerprint identification is infallible." Name the logical move this commits (the chapter calls it out by structure), and use the snowflake analogy to explain why uniqueness of the source does not establish reliability of the comparison.

  3. A television lab "runs the print" and a photograph of the suspect appears on screen in nine seconds. Using §14.5, give two specific ways this is backward from how AFIS and ACE-V actually work.

  4. A report concludes "14-point match — identification" but never states the quality of the latent, who decided the points agreed, or whether the verifier was blind. List the questions a careful reader (or cross-examiner) should ask before trusting the conclusion, and explain why each matters.

E. Ethics and reasoning

  1. † An examiner is told, before beginning, that "the suspect already confessed and the detective is certain it's him," and is handed the suspect's exemplar at the same time as the latent. Using the Cognitive-Bias Watch in §14.4 (and previewing Chapter 31), explain two distinct bias entry points this creates and the procedural fix (linear ACE-V / sequential unmasking) for each.

  2. A lab's verification policy lets the second examiner see the first examiner's conclusion ("IDENT") on the case file before re-examining. Explain why this makes verification "confirmation," and why a conclusion that two examiners agree on is not automatically twice as trustworthy if the second knew the first's answer.

  3. You are asked to testify that a fingerprint identification is "one hundred percent certain." Explain why you should decline, what category error "certainty" commits about a human judgment, and what you can honestly say instead.

  4. The 2011 FBI/Noblis-type error studies found false positives rare (a fraction of a percent) and false negatives considerably more common. Explain why "a fraction of a percent" is reassuring about the typical case and not a license for the word "zero," and what the asymmetry tells you about how conservative examiners tend to be.

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); latent fingerprint comparison (this chapter); microscopic hair comparison (Chapter 19, previewed); bite-mark comparison (Chapter 16, previewed). State specifically what "foundationally valid but with a non-zero error rate" means for fingerprints.

  2. Explain how the autopsy finding that placed the cold-case death as a homicide (Chapter 11) and the gas-can latent's inconclusive result (this chapter's Case File) are both consistent with the book's first theme — "forensic science excludes more reliably than it proves." What did each result establish, and what did each decline to claim?

  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.

G. Cold-case extension

  1. † Cold Case. The gas-can latent came back inconclusive even though Roy Keller appeared on the AFIS candidate list and a DNA mixture (Chapter 9) was consistent with him. Write the entry you would add to the Mill Creek evidence log (Appendix I). State (a) the defensible conclusion at its true strength, (b) the honest verb (or the honest refusal of a verb), (c) at least three things this latent specifically does not establish, and (d) why, in one sentence, declaring "Keller's print, on the gas can" from this latent would have been "Madrid in miniature."

  2. Cold Case extension. Name the specific safeguards the cold-case examiner applied that the Mayfield examiners did not, and explain how each safeguard maps onto a failure mode from Figure 14.1. Then state honestly whether the inconclusive result advances or retreats the case against Keller, and why "neither" is the correct answer.

  3. Cold Case, integrative. The latent is "of limited value," and the DNA mixture (Chapter 9) is "the stronger, still-not-conclusive thread." List two other evidence types from earlier chapters that, combined, would begin to place Keller's hand or person at the cabin — and state plainly why the inconclusive print can do none of that work on its own.

H. Short writing

  1. In 150–200 words, explain to a juror why a fingerprint identification is genuinely useful evidence and why it is never the same thing as "the science proves this is his print, with certainty." Use the Mayfield case as your single concrete example.

  2. † In 150–200 words, contrast the biological basis of fingerprints (which is sound) with the interpretive method of comparing partial latents (which carries a real, non-zero error rate). Explain why conflating the two — "the ridges are unique, therefore the examiner is infallible" — is the chapter's central error, and name where on the validity spectrum the honest version of the method actually sits.