Chapter 24 — 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 gunshot residue (GSR) and primer residue. Which three elements does the diagnostic primer particle most commonly contain, and what two things (not one) must be true of a particle for it to be confidently identified as primer residue?

  2. † Why is "lead present on the swab" a far weaker finding than "a single particle containing lead, barium, and antimony fused together"? Name two everyday, non-firearm sources of lead, barium, or antimony to make the point.

  3. Define the refractive index of glass. Why is it an excellent tool for excluding a common source but only a weak tool for associating two fragments with a common source?

  4. Define paint-layer comparison. In one sentence, state where the probabilistic weight of a paint comparison actually lives — in a single layer, or somewhere else?

  5. Define soil comparison. List four distinct properties of soil that can serve as independent comparison points.

  6. What instrument from Chapter 23 is central to identifying primer-residue particles, and what two kinds of information does it give about a single particle simultaneously?

  7. Name the two families of cracks that form when a pane is broken by an impact, and state which radiates outward from the point of impact and which forms rings around it.

  8. What is a physical (fracture) fit, and why is it the one circumstance in which paint or glass evidence approaches individualization rather than class evidence?

B. Applied reasoning

  1. † A suspect is sampled for GSR at the police station, three hours after the shooting, after a ride in a patrol car and time in a holding cell, and a positive result is reported. Identify three separate reasons this positive result is weaker than it sounds, and state what an honest analyst can and cannot conclude from it.

  2. A person who fired a weapon is sampled for GSR eight hours later, after a full day of ordinary activity, and the result is negative. Explain why this negative result proves very little, and why GSR's absence is weak evidence in both directions.

  3. Two automotive paint comparisons each "match." In Case A the agreement is a single layer of common white paint; in Case B it spans six layers, including a buried non-factory repaint in an unusual color. Both are class evidence — so explain, using distinctiveness and independence/number, why Case B is a far stronger association.

  4. † A glass fragment from a suspect's shoe has a refractive index that differs from the broken window at the scene. What can you conclude, and is it an exclusion, a consistency, or an individualization? Contrast this with what you could conclude if the refractive indices had been indistinguishable.

  5. An investigator collects a single soil sample from the center of a large lot to compare against soil on a suspect's boots. Explain what is wrong with this collection strategy and what samples should have been taken instead, using the ideas of distinctive soil and background/control samples.

  6. A pane at a burglary scene shows the microscopic stress marks of a radial fracture. Explain, in your own words, how an analyst uses these marks to determine which side the force came from — and why that determination bears on whether a break-in or a staged scene is more plausible, without naming who broke the glass.

  7. Layers of two visibly different soils are found on a single shovel. What can this stratigraphy potentially reveal that a single homogenized scrape could not? Relate it to the layer-reading logic of a paint chip.

C. Evidence interpretation

  1. † Re-read Figure 24.1 ("A paint chip in cross-section"). The buried blue refinish layer beneath the red basecoat is doing specific work in the inference. Explain what that layer adds to the strength of the association, and write the single honest sentence an examiner could say about the five-layer agreement on the stand.

  2. Re-read Figure 24.2 ("Reading a broken pane"). List two things the fracture pattern can establish and two things it cannot. Then state the honest verb for the directional conclusion.

  3. Re-read Figure 24.3 ("Soil in the boot tread") — the cold-case figure. The figure concludes the soil "strongly supports" the boots having been in the cabin's distinctive environment. (a) What specific fact about the control samples earns the word "strongly" rather than merely "consistent with"? (b) Write the single sentence an honest expert could say about this evidence on the stand, and the single sentence that would overstate it.

  4. A GSR examiner reports: "Three particles consistent with primer residue, of the highest confidence class, were present on the sample; the presence of such particles is consistent with the person having discharged, handled, or been in proximity to a fired firearm, or with contact transfer, and does not by itself establish which." Identify three separate things this sentence does well (where it is appropriately honest).

D. Spot the overstatement / junk-science alert

  1. † A prosecutor's slide reads: "Forensic analysis proves the soil on the defendant's boots came from the murder scene." Identify two distinct overstatements in this sentence and rewrite it honestly.

  2. An examiner testifies that two glass fragments "match," and the jury is left to draw its own conclusion. Explain why the bare word "match" is the most dangerous word in this chapter — what does the examiner mean by it, and what does the jury hear?

  3. Under cross-examination, a trace examiner is pushed to quantify the strength of a soil association and answers, "There's about a one-in-a-million chance this soil came from anywhere else." Assuming no validated soil-population database exists, explain why this is a fabrication even if offered in good faith, and what the examiner should have said instead.

  4. A television detective swabs a suspect's hands, the "GSR test" comes back positive, and the suspect is declared the shooter on the spot. Using §24.1, give two reasons this is backward from how the evidence actually works.

  5. An expert testifies that a single layer of common black automotive paint on a victim's clothing "links the defendant's car to the crime." Name the specific overstatement (what is being claimed that a single common layer cannot support) and give the defensible version.

E. Ethics and reasoning

  1. † A GSR particle examiner is told by the detective, before analysis, "We're confident this is our shooter — we just need the GSR to confirm it." Using the Cognitive-Bias Watch in §24.1 (and previewing Chapter 31), explain (a) how this information could bias an otherwise honest particle examination, and (b) the safeguard. Note specifically where in the GSR process the contamination usually happens — before or after the sample reaches the lab — and why that matters.

  2. Two qualified examiners compare the same paint chip; one calls the association "strongly supports a common origin," the other "consistent with a common origin." Is this necessarily a scandal, or can it be a legitimate difference in how each weighed distinctiveness and the number of matching layers? Explain, and say what would make it a genuine error.

  3. You are asked to testify that a soil match "individualizes" the crime scene the way DNA individualizes a person. Explain why you should decline, and what you can honestly say instead about a distinctive soil with good controls.

  4. Why is refusing to invent a frequency under cross-examination an act of integrity rather than weakness? Connect your answer to the prosecutor's-fallacy concerns of Chapter 9 and to the validity-spectrum argument of this chapter.

F. Synthesis and validity spectrum

  1. † Place these four kinds of evidence on the NAS 2009 / PCAST 2016 validity spectrum (strong → discredited), justifying each: single-source nuclear DNA (Chapter 7); a multi-layer paint comparison (this chapter); a single-layer common-paint comparison (this chapter); bite-mark comparison (Chapter 16, previewed). What feature determines where each falls?

  2. Explain how the negative GSR result in the cold case (this chapter), the no gunshot wound autopsy finding (Chapter 11), and the red-herring cartridge case (Chapter 15) all sit together consistently. Which of the book's four themes does this convergence most clearly illustrate?

  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. Using only what the soil-on-the-boots evidence establishes, write the entry you would add to the Mill Creek 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 you decline to convert "places Keller's boots in the cabin's environment" into "Keller was at the scene when Diallo died" or "Keller is the killer."

  2. Cold Case extension. Roy Keller is a co-owner of the cabin property. Explain precisely how this single fact bears on the weight of the soil association — what it does and does not weaken — and why it is exactly the kind of innocent-explanation context an honest report must include alongside the finding.

  3. Cold Case, integrative. The soil "places Keller (via his boots) in the cabin's environment" but not at a time and not as a culprit. List two other evidence types from earlier chapters that, combined with the soil, would begin to address the questions soil cannot — when he was there and whether a person (not just his boots) is tied to the crime — and state plainly why the soil alone cannot make those leaps. (Think Chapters 13, 25, 7–9, 27.)

  4. Cold Case. The GSR result in this chapter is negative. Write two sentences: one stating what that negative result corroborates and why it sits consistently with earlier findings, and one stating the §24.1 caution about why a negative GSR would prove little if it were asked to stand alone.

H. Short writing

  1. In 150–200 words, explain to a juror why "gunshot residue was found on the defendant" is not the same thing as "the defendant fired the gun" — and why that gap is the heart of the GSR contamination problem.

  2. † In 150–200 words, contrast the validity foundations of glass refractive-index comparison and soil comparison: what does each rest on, where is each strong (exclusion vs. association), and where is each most easily overstated in court?

  3. In 150–200 words, explain the principle that runs through all four materials in this chapter — that the strength of a class association scales with the rarity (distinctiveness × number of independent features) of the match — using one example each from paint, glass, and soil.