Chapter 3 Exercises: Physical Evidence

These exercises move from recall to applied reasoning to ethics. Work them in order; the later ones assume the vocabulary built by the earlier ones. Items marked with a dagger (†) have worked solutions in the answers appendix. There are no answers in this file — that is deliberate. Write your reasoning out; the point is the argument, not the label.

A note on honest verbs. Several items ask you to "state what the evidence establishes." Use the chapter's ladder — excludes → cannot exclude / consistent with → strongly supports → proves — and climb only as far as the evidence carries you. An answer that says "proves" will almost always be wrong.

Part A — Recall and definitions (warm-up)

3.1. State Locard's exchange principle in one sentence. Then explain, in one more sentence, why the slogan "every contact leaves a trace" is incomplete — what does it leave out about the direction of transfer?

3.2.† Define each term in one sentence and give one concrete example of each: (a) transient evidence, (b) conditional evidence, (c) pattern evidence, (d) transfer evidence, (e) associative evidence.

3.3. What is the difference between a questioned sample and a known/reference sample? Which one's defining feature is that you are certain where it came from?

3.4. Name the four kinds of control sample described in the chapter (substrate, reagent/negative, positive, elimination) and state, in a phrase, what each one rules out or makes possible.

3.5.† List the four real limits of Locard's principle — the four things that can go wrong even though a trace was genuinely deposited.

3.6. Why is biological evidence packaged in breathable paper rather than airtight plastic? Name the one category of evidence for which the rule reverses (airtight, not paper), and say why.

3.7. Distinguish degradation from contamination. Which one adds foreign material, and which one is the sample decaying on its own?

3.8. In one sentence each, name two things physical evidence can establish and two things it cannot.

Part B — Classify and apply

For each item below, (i) name the evidence category or categories from §3.2 that best fit, and (ii) state the single most important thing about handling it (collect immediately? document only? package which way?).

3.9. A strong smell of bleach in a bathroom where a body was found.

3.10.† A muddy shoe impression in soil at the edge of a driveway.

3.11. The interior temperature of a car's engine hood, felt by the first officer on scene.

3.12. A single bright-blue acrylic fiber clinging to the victim's collar.

3.13. A spray of small bloodstains across a wall, fanning upward and outward.

3.14. A front door found locked from the inside, with an intact chain, in a "burglary" report.

3.15.† A metal can of fire debris collected from the floor of a burned room, suspected to contain an accelerant.

3.16. A wallet belonging to a known individual, dropped on the floor near the point of entry.

Part C — Controls, comparison, and interpretation

3.17. You recover an unknown reddish-brown stain from a painted wooden windowsill and intend to test it for blood and then for DNA. List, in order, the questioned sample, the known/reference sample(s), and the control sample(s) you should collect, and say what each one contributes.

3.18.† A lab reports that a presumptive test for blood on a recovered cloth came back negative. Before you accept "there was no blood here," what control would you insist was run, and what would a failure of that control mean for the negative result?

3.19. Explain why an unidentified, complete, high-quality DNA profile recovered from a scene is of limited immediate value — and what single thing would convert it into a strong association. (Connect this to the role of known samples.)

3.20. A reference fiber sample is taken from one cuff of a large, multicolored sweater, but the part of the sweater that actually contacted the victim was a different color. Explain how this makes the reference sample inadequate, and what the collector should have done differently.

3.21.† Two evidence items — a bloodstained shirt and a knife — are collected at a scene and, to save bags, placed in the same large paper sack for transport. Name the specific risk this creates, which threat to evidence integrity it falls under, and the rule that was violated.

3.22. A first responder turns off a kitchen light "to take a better photo with the flash" before the scene is documented. Using the concept of conditional evidence, explain exactly what may have been destroyed and why it cannot be recovered.

Part D — Spot the overstatement

Each statement below climbs the verb ladder too high or asks evidence to leap its category. Identify the overstatement and rewrite the sentence so it is defensible.

3.23.† "The suspect's fingerprint was on the windowsill, so he is the burglar."

3.24. "His DNA was on the knife handle, which proves he stabbed the victim."

3.25. "Gunshot residue was found on her hands, so she fired the weapon."

3.26.† "The bloodstain reconstruction shows the defendant beat the victim and then set the fire."

3.27. "The pry marks on the door match the suspect's pry bar, so he broke in."

3.28. "We found the victim's fibers in the suspect's car, so the victim was in that car the night of the murder."

Part E — Reasoning and ethics

3.29. Locard's principle "guarantees the trace exists." A junior investigator argues: "Then if we search hard enough, we are guaranteed to connect the suspect to the scene." Identify the logical error, and connect it to the Cognitive-Bias Watch in §3.1.

3.30.† A detective tells the lead CSI, before processing begins, "We know it's the business partner — just find me something tying him to that gas can." Describe two distinct ways this single sentence threatens the integrity of the physical-evidence analysis, drawing on both §3.1 and §3.6, and name the safeguards.

3.31. Reconstruction "names a sequence, not a suspect." Explain what is dangerous about a vividly narrated courtroom reconstruction ("he stood here, struck her there"), and why a jury may find it more persuasive than the underlying evidence warrants.

3.32. You are the analyst. A substrate control you collected comes back positive for DNA — meaning the surface itself, not just the stain, carried genetic material. The detective wants you to "just report the stain result and not complicate things." Explain why the control result must be reported, and what it does to the interpretability of the stain.

3.33.† An item arrives at the lab in a sealed paper bag, but the tamper-evident tape across the seal is signed but not dated, and there is a six-day unexplained gap in the chain-of-custody log. The physical sample looks perfect. Explain why "the sample is fine, so the gap doesn't matter" is the wrong conclusion, distinguishing physical integrity from documentary integrity.

Part F — Cold-case extension

3.34. Return to the Mill Creek inventory in this chapter's Case File (the gas can, the tools, the charred documents, the pry-marked door, the phone, the cartridge case). For each of the six items: (i) name its evidence category, (ii) state the single most that it could honestly establish at this stage, and (iii) name the one thing it cannot establish that a careless investigator might assume it does.

3.35.† The cabin door bears pry marks and was found at a fire scene processed late. Using the concepts of conditional evidence and evidence integrity, write a short paragraph explaining (a) why the door's value depended on whether its state was documented before it changed, and (b) what specific facts about the pry marks the physical evidence can and cannot supply. Then state the honest "status after Chapter 3" line in your own words.

3.36. The stray cartridge case is logged as "associative potential." Knowing nothing yet about a wound or a weapon, write two sentences: one stating what this item could conceivably become relevant to, and one explaining why it is, for now, an item of unknown relevance rather than evidence of a shooting. (You are foreshadowing a later lesson about relevance — resist concluding.)