Chapter 39 — Exercises

This is the capstone, so these exercises ask you to assemble and weigh, not merely recall. Work them without looking back first; then check yourself against the chapter. Items marked † have full worked solutions in the answers appendix. There are no answers in this file. The set mixes recall, applied assembly, evidence interpretation, "spot the overstatement," ethics, and a culminating cold-case capstone task (Exercise 39.40).

A. Recall and definitions

  1. Define forensic case assembly in one sentence, and state the first rule of honest assembly (what happens to a thread's strength when it gains company).

  2. Define exclusion matrix and explain, in one sentence each, why it embodies the book's first theme.

  3. † Define convergence of evidence, and identify the single word in your definition that does the load-bearing work. Explain why.

  4. Define weight of evidence, and place the four honest verbs of this book in order from weakest to strongest, marking the one verb that is forbidden.

  5. Name the three persons of interest the science excluded in the Mill Creek case and, for each, the evidence that excluded them.

  6. What does it mean that Roy Keller's overall status in the matrix is "not excluded" rather than "guilty"? Why is the distinction essential?

  7. Name the three legs of the classic circumstantial triad and, for each, one Mill Creek thread that fills it.

  8. What is the "unknown-stranger theory," and which chapter's method closed it? Why must an honest assembly close it before converging on anyone?

B. Applied assembly and reasoning

  1. † Reconstruct the exclusion of Cody Renner. List the three independent findings that excluded him, and explain why his confession did not end the investigation. What does this row of the matrix teach about the reliability of confessions relative to physical and digital evidence?

  2. The chapter says convergence carries weight only because the threads are independent. For each of the following Mill Creek pairs, state whether the two threads are genuinely independent and why it matters: (a) the GC-MS gasoline confirmation and the bank records; (b) the cell-site location and the soil on the boots; (c) the way the original analysts described the DNA mixture and the way they described the cell-site sector.

  3. Explain, using the §39.3 illustrative multiplication, why independent improbabilities multiply but non-independent ones do not. Why is presenting a multiplied figure to a jury dangerous even when the threads are independent?

  4. † The autopsy finding (no soot in the airways, Chapter 11) and the engineering finding (electrical system ruled out, Chapter 36) both attack the "accidental fire" explanation, but from different directions. Explain how each one closes the accident theory, and why having both is stronger than having either alone.

  5. Build the motive leg of the convergence in your own words, citing the two chapters that establish it, and then state precisely why "strong motive" is not "guilt."

  6. Build the opportunity leg, listing each thread and the honest verb attached to it. For each thread, name the specific upgrade a careless reader might wrongly make (e.g., "phone in the area" → "man at the scene").

  7. Build the means leg, and identify which single piece of opportunity evidence also reinforces means (hint: a purchase on camera).

C. Evidence interpretation and the honest verb

  1. † For each Mill Creek thread, give the strongest honest verb (excludes / consistent with / strongly supports) and one limit: (a) the touch-DNA mixture on the gas can; (b) the pry marks on the door; (c) the soil on Keller's boots; (d) the cell-site records; (e) the dental identification of the body.

  2. The dental identification (Chapter 17) is the one biological thread in the file that approaches certainty. Explain why it sits higher on the validity spectrum than the DNA mixture, even though both are "biological."

  3. Why is the soil evidence weaker against Keller specifically than it would be against a stranger? Name the single fact about Keller that cuts against over-reading the soil.

  4. The GSR test was negative. Explain why a negative result is "consistent with" the case rather than evidence for Keller, and connect it to the red-herring cartridge case (Chapter 15).

  5. † A draft report concludes: "The forensic evidence proves beyond doubt that Roy Keller murdered Marcus Diallo." Identify every overstatement in that sentence and rewrite it at the strength the evidence actually supports.

D. The defense's case and residual uncertainty

  1. State, at its strongest, the defense argument that the DNA mixture does not establish Keller's contribution to a certainty. What is the honest prosecution response that neither concedes too much nor overstates?

  2. The toolmarks are a class match. Construct the defense's argument from that fact, and explain why "class evidence places, it does not individualize" is a legitimate point rather than a technicality.

  3. † Explain how a compromised scene (Chapter 2) widens the uncertainty around every physical thread collected afterward. Does it erase the convergence? Defend your answer.

  4. The original analysis was biased by the accidental-fire frame (Chapter 31). Construct the defense's argument from that fact, and then state the prosecution's best response (hint: which threads are hardest to bias, and why).

  5. List four further tests that would strengthen the Mill Creek conclusion. For each, state how the same test could instead weaken it, and explain why that symmetry is the mark of honest science.

E. "Spot the overstatement" and the themes

  1. A prosecutor's closing says: "Twelve separate pieces of forensic evidence all point to one man — the math is overwhelming." Identify the two hidden assumptions in "the math is overwhelming," and explain when, if ever, the sentence is defensible.

  2. † A commentator says: "The case only 'strongly supports' Keller, so it's basically the same as the bite-mark cases that convicted innocent people." Refute this using the distinction between an excluded field plus independent convergence and a single overstated inclusion. Which theme does the commentator misunderstand?

  3. Explain how the criminal profile (Chapter 28) is treated in the assembly, and why giving it zero weight — even though it is "forensic" — is the correct application of Theme 2.

  4. The chapter says the CSI effect "cuts both ways" even at the capstone. Give the two opposite errors a juror could make about a "strongly supports" conclusion, and the calibrated middle position.

  5. Which of the four themes does each of the following most directly illustrate: (a) excluding Salas despite his motive; (b) refusing to weigh the profile equally with the DNA; (c) demanding the threads be independent; (d) refusing both "it proves it" and "it's not enough"?

F. Ethics and the courtroom

  1. † You are the analyst. The lead detective asks you to testify that the evidence "proves" Keller did it, because "the jury needs to hear certainty." Draft your response, grounded in the ultimate-issue problem (Chapter 30) and the duty to the court.

  2. A defense expert is retained to re-analyze the DNA mixture. The prosecution objects that this is "just hired-gun obstruction." Explain why independent (ideally blind) re-analysis is exactly what should happen, and what it protects against.

  3. Suppose a blind re-analysis (Chapter 38) failed to reproduce one of the converging threads. Explain why the chapter calls that "the most important finding of all," and what you would do with the rest of the assembly.

  4. An honest assembly can "name, out loud, everything its own conclusion does not establish." Explain why that ability is the surest sign of competence rather than weakness, and give three items from the Mill Creek "does not establish" list.

  5. Write the single sentence an expert may honestly say on the stand about the totality of the Mill Creek evidence, and the single sentence they must refuse to say. Explain the difference in one line.

G. Cumulative (across the book)

  1. Trace one thread — the gas can — across every chapter that touched it (the inventory, the touch-DNA, the mixture, the latent print, the CCTV purchase). State what each chapter added and the honest verb at each step. Why is the gas can a good illustration of "one brick, not the wall"?

  2. † Compare the role of exclusion in the Mill Creek case to its role in the Kirk Bloodsworth exoneration (Chapter 6). What is the same about the logic, and what is different about the direction it ran?

  3. The book opened and closed with Sherlock Holmes. Explain, using the capstone, why "the real forensic scientist assembles the answer and refuses the leap" is the opposite of the Holmes method — and why the difference matters for justice.

  4. Place the whole assembly on the validity spectrum: identify the strongest-foundation method used in the case and the weakest, and explain how the assembly weighted them differently. Why would a case built by treating all "forensic" threads as equal be junk?

H. The culminating cold-case capstone task

  1. The Final Report. Using the workbook templates in Appendix I, write a complete, honest forensic case report on the Mill Creek case, suitable for handing to a prosecutor who must decide whether to charge. Your report must contain, in order: - (a) The question. State the original question of the case in one sentence. - (b) The evidence log. A table of every evidence type, its source chapter, its honest verb, and its key limit. - (c) The exclusion matrix. Reproduce or rebuild it; for every excluded person, give the reason for exclusion. - (d) The convergence. Lay out the motive/opportunity/means/biological-anchor structure, with the honest verb on each thread, and a one-paragraph argument for why the convergence carries weight (the independence point). - (e) The conclusion. State the weight of evidence at its true strength — strongly supports, not proves — in a single, defensible sentence. - (f) The residual uncertainty. A bulleted list of everything the conclusion does not establish, including the mixture limit, the class-not-individual toolmarks, the place-not-person trace evidence, the compromised scene, and the original bias. - (g) Further testing. A bulleted list of tests that would strengthen — or could break — the conclusion, with the symmetry made explicit for at least two. - (h) A self-audit. One paragraph: where in your own report were you most tempted to overstate, and how did you resist it? Which of the four themes did that resistance serve?

Then write a one-paragraph reflection: having assembled the whole case, what is the single most important difference between the conclusion you reached and the conclusion a television detective would have announced — and why is your weaker-sounding conclusion the stronger one?