Chapter 3 Quiz: Physical Evidence

Twenty-five questions — multiple choice and short answer — to check your grasp of the chapter before moving on. Answer first, then open the key at the bottom. Short-answer questions are graded on whether your reasoning matches the chapter's, not on exact wording.

Multiple choice

Q1. Locard's exchange principle is best stated as: A. Every crime leaves a fingerprint. B. Whenever two objects contact, there is a mutual transfer of material between them. C. A trace recovered at a scene always identifies the perpetrator. D. Contact between two surfaces destroys all prior evidence.

Q2. Which of the following is the most accurate statement about Locard's principle? A. It guarantees that any deposited trace will be found if examiners search carefully. B. It guarantees a trace is deposited but not that it will be detected, persist, or be interpretable. C. It applies only to biological evidence. D. It proves that physical evidence is always present at violent crimes.

Q3. A gasoline odor noticed by the first firefighter inside a burning structure is an example of: A. Pattern evidence. B. Associative evidence. C. Transient evidence. D. Individual-characteristic evidence.

Q4. A front door found with its deadbolt thrown into a splintered frame, recording the state of entry at the time of the event, is best classified as: A. Transient evidence only. B. Conditional (and pattern) evidence. C. Reference evidence. D. A control sample.

Q5. The defining feature of a known/reference sample is that: A. It is collected from the scene. B. Its source is certain and documented. C. It is always biological. D. It is collected last.

Q6. You swab a bloodstain off a painted windowsill. To show the paint itself is not the source of any DNA signal, you also swab an adjacent unstained area of the same sill. This is a: A. Positive control. B. Reagent control. C. Substrate (background) control. D. Elimination sample.

Q7. Reference samples taken from the homeowner and the responding officers, so their DNA or prints can be set aside rather than mistaken for a suspect's, are called: A. Substrate controls. B. Positive controls. C. Questioned samples. D. Elimination samples.

Q8. A presumptive blood test reported as negative is only trustworthy if which control showed the test could have detected blood? A. Substrate control. B. Positive control. C. Elimination sample. D. Reference fiber.

Q9. Which is the strongest and cleanest thing physical evidence typically does? A. Prove guilt. B. Establish intent. C. Exclude a suspect or source. D. Fix the time of contact.

Q10. "His DNA was on the doorknob" most directly establishes: A. That he committed the crime. B. That his cells are on the doorknob (source of the cells), but not when or how they got there. C. That he was at the scene at the time of the crime. D. His intent.

Q11. Bloodstains found beneath a char line on a doorframe most defensibly establish: A. Who deposited the blood. B. That the blood was deposited before the fire (a sequence). C. That a homicide occurred. D. The time of death by the clock.

Q12. Which of the following is something physical evidence generally cannot establish? A. That a substance is gasoline. B. That two profiles do not match (exclusion). C. The intent or state of mind of an actor. D. That a sequence of events occurred in a certain order.

Q13. Biological evidence should be packaged in: A. Airtight plastic, to seal out contaminants. B. Breathable paper, after air-drying, to prevent moisture-driven decay. C. Any container, since DNA is durable. D. The same bag as related items, to keep the case together.

Q14. The packaging rule reverses (airtight container required) for: A. Bloodstained clothing. B. A buccal swab. C. Fire debris suspected to contain a volatile accelerant. D. A latent fingerprint on paper.

Q15. Contamination is uniquely dangerous because it can: A. Only weaken an otherwise valid result. B. Manufacture an association that never existed (e.g., a real but irrelevant profile on a swab). C. Always be removed later in the lab. D. Be detected without any controls.

Q16. A single tool (e.g., forceps) used to collect two different evidence items, without cleaning between, risks: A. Degradation of both items. B. Cross-transfer of material from one item to the other (contamination). C. Improving the comparison. D. Nothing, as long as the tool is metal.

Q17. A reconstruction from physical evidence can most defensibly establish: A. The identity of the actor. B. The motive. C. A sequence/order of events. D. The time of the crime to the minute.

Q18. The rule "reconstruction names a sequence, not a suspect" means: A. Reconstruction is useless in court. B. Reconstruction can establish what happened in what order, but adding an actor requires other evidence. C. Reconstruction always identifies the perpetrator. D. Sequences cannot be determined from physical evidence.

Q19. Most physical evidence carries class characteristics, which means it: A. Points to one unique source. B. Associates an item with a group (e.g., a fiber type, a shoe brand and size). C. Has no evidentiary value. D. Cannot be compared.

Q20. Which combination correctly pairs an evidence type with the threat its packaging must guard against? A. Biological evidence → trapped moisture (use paper). B. Biological evidence → evaporation (use airtight cans). C. Fire debris → trapped moisture (use paper). D. Liquid blood → drying (use paper).

Short answer

Q21. Give the four real limits of Locard's principle — the four reasons a deposited trace may still fail to help a case.

Q22. Explain the difference between a substrate control and a positive control, and say what each one tells you.

Q23. A detective claims, "We found the suspect's fibers on the victim, so the suspect attacked the victim." Identify two distinct overstatements in that sentence and rewrite it so it is defensible.

Q24. Why is documentary integrity (chain of custody, sealed and dated packaging) as important as physical integrity? Describe a scenario where the sample is physically perfect but its value is still compromised.

Q25. Using the cold-case inventory, explain what the gas can's potential touch-DNA and latent print could — at most — establish at this stage, and name one thing a careless investigator might wrongly assume they establish.

Answer key (click to expand) **Q1.** B — mutual transfer between contacting objects. **Q2.** B — it guarantees deposition, not detection/persistence/interpretability. **Q3.** C — an odor is transient; it must be documented, not collected. **Q4.** B — the lock state is conditional; the splintering pattern is also pattern evidence. **Q5.** B — a known/reference sample's defining feature is a certain, documented source. **Q6.** C — an adjacent unstained swab of the same surface is the substrate (background) control. **Q7.** D — elimination samples, from people with legitimate access. **Q8.** B — a positive control shows the test *can* detect the target; without it, a negative is uninterpretable. **Q9.** C — exclusion is forensic science's strongest, cleanest voice. **Q10.** B — it establishes the source of the cells, not when/how they arrived or guilt. **Q11.** B — blood beneath the char establishes a *sequence* (deposited before the fire), nothing more by itself. **Q12.** C — intent/state of mind is a legal inference, never an instrument's finding. **Q13.** B — breathable paper after air-drying; moisture breeds the bacteria/mold that destroy DNA. **Q14.** C — volatile fire debris must be sealed airtight so the accelerant vapor cannot escape. **Q15.** B — contamination can create a real-but-irrelevant association; it is often invisible and unrecoverable. **Q16.** B — cross-transfer between items is a classic, avoidable contamination. **Q17.** C — a sequence/order of events. **Q18.** B — sequence is defensible; adding an actor requires other evidence. **Q19.** B — class characteristics associate an item with a group, not a unique source. **Q20.** A — biological evidence's threat is trapped moisture, so use breathable paper. (Fire debris is the reverse: airtight to trap volatiles; B, C, and D mismatch the threat.) **Q21.** (1) The trace may be too small/sparse to detect; (2) it may not persist (transfer ≠ persistence); (3) it may be undetectable against its background (e.g., white cotton on white cotton); (4) even when recovered and read, it usually establishes association, not the actor, time, or intent. **Q22.** A *substrate control* is unstained material from the same surface as the questioned stain — it shows the surface itself is not the source of the signal. A *positive control* is the test run on a sample known to contain the target — it shows the test is working and *would* detect the target if present (making a negative result meaningful). **Q23.** Overstatement 1: from *association* to *identity/action* ("attacked") — fibers associate materials, not actions. Overstatement 2: ignores *timing* — the transfer carries no timestamp and could be innocent or secondary. Defensible: "Fibers consistent with the suspect's garment were recovered from the victim; this is consistent with contact between the two, though it does not establish when, how, or by whose action the transfer occurred." **Q24.** Documentary integrity proves the physical sample is the same item, unchanged and untampered, from scene to court. A physically perfect sample with a broken chain (unexplained gap, unsealed or undated tape, missing signature) can no longer be *vouched for* — its connection to the scene and its freedom from tampering are now in doubt, which a defense will rightly attack. Example: a pristine bloodstained shirt whose evidence bag was found unsealed in an unlogged location for several days. **Q25.** At most, the gas can's touch DNA and latent print could establish *whose cells/finger contacted the can's handle* — an association between a person and the object — and possibly help *exclude* people who do not match. They cannot establish *when* the contact occurred, that the person *set the fire*, or *intent*. A careless investigator might wrongly assume "his DNA on the can = he set the fire," collapsing association into action and ignoring timing.