Chapter 10 — Exercises

Work these after reading the chapter. They run from recall to applied reasoning to ethics and evidence interpretation. Items marked with a dagger (†) have worked solutions in the answers appendix; the rest are for discussion or self-check. No answers appear in this file. Honest verbs matter: prefer excludes, consistent with, strongly supports over proves.

A. Recall and definitions

  1. Define serology in one sentence, and state the two questions modern serology answers well.

  2. † Distinguish a presumptive test from a confirmatory test. For each, give one example used on blood and state what a positive result does and does not license you to conclude.

  3. Name the molecule, and the specific part of it, that every common presumptive blood test actually detects. Why does this explain the tests' false positives?

  4. What color change indicates a positive Kastle-Meyer test, and what is the significance of the timing of the color change relative to adding hydrogen peroxide?

  5. List three substances other than human blood that can produce a luminol false positive.

  6. Define spatter, and explain why the word describes a physical observation rather than a conclusion about cause.

  7. Define area of origin, and explain the difference between the area of convergence (2-D) and the area of origin (3-D).

  8. † State the geometric relationship between a stain's width-to-length ratio and its angle of impact, and explain in plain words why a long, thin stain implies a shallow angle.

  9. What is the difference between a passive stain, a transfer/contact stain, and a spatter stain? Give one example of each.

  10. Why do red blood cells contribute no nuclear DNA, and why is whole blood nonetheless one of the richest sources of DNA at a scene?

B. Applied reasoning

  1. A CSI finds twelve reddish marks in a kitchen. Explain, using the two-stage funnel, how presumptive testing lets them decide which marks to collect for DNA without testing all twelve fully.

  2. † A bathroom that a suspect is known to have cleaned with bleach gives a strong, even luminol glow across the entire floor. An investigator testifies this proves "the defendant cleaned up a bloodbath." Identify every overstatement in that testimony, and write the honest report line the luminol result actually supports.

  3. A dried bloodstain is on a painted wall. Write the ordered collection steps a CSI should follow, and justify why each step comes when it does. Include what controls are taken and why.

  4. † A bloody T-shirt is recovered, sealed wet in a plastic evidence bag, and arrives at the lab three days later in summer. Predict the likely condition of the DNA, name the packaging rule that was violated, and explain the chemistry/biology of why the rule exists.

  5. An analyst measures a stain at 4 mm wide and 8 mm long. Estimate the angle of impact. Then list three reasons your estimate carries error even though the formula is exact.

  6. A spatter pattern is found low on a wall. The area-of-origin reconstruction places the source about 35 cm above the floor. State one account of events this is consistent with, one it is difficult to reconcile with, and one thing the reconstruction does not establish.

  7. Explain why the straight-line "stringing" method tends to place the area of origin too high, and what physical fact it ignores.

  8. † Two qualified BPA analysts examine the same stains on a shirt. One calls them high-velocity impact spatter (placing the wearer at a shooting); the other calls them transfer stains (innocent contact afterward). What does this disagreement reveal about where BPA sits on the validity spectrum, and what should a jury be told about it?

  9. A detective tells a BPA analyst, "The husband claims she fell down the stairs, but we think he beat her — read the blood and confirm it." Identify the cognitive-bias hazard by name, the safeguard that should have been in place, and why the analyst's resulting interpretation is now compromised even if it happens to be correct.

  10. A stain is presumptively positive (Kastle-Meyer pink) but a careful confirmatory species test indicates it is not human blood. Walk through what each result means and what the investigator should conclude.

C. Evidence interpretation ("Read the Evidence")

  1. † Construct a six-field Read the Evidence block (THE ITEM / THE CONTEXT / WHAT IT SHOWS / WHAT IT DOESN'T / THE INFERENCE / THE LESSON) for the following constructed teaching example: a single elongated bloodstain, 3 mm wide and 15 mm long, on a vertical wall, with a small tail pointing upward and to the right. State only what is defensible.

  2. Given a photograph (described): a large, roughly circular pool of blood with smooth edges on a tile floor, with no satellite droplets. Classify the stain category and state two reasonable and two unreasonable inferences.

  3. A wall shows dozens of tiny (sub-millimeter) blood droplets densely clustered in one region. State what the size of the droplets loosely suggests about the energy of the event, and three different mechanisms that could produce such a fine mist — explaining why the pattern alone cannot choose among them.

D. Spot the overstatement

  1. For each statement, mark whether it is defensible or an overstatement, and rewrite the overstatements honestly: (a) "Kastle-Meyer was positive, so this is the victim's blood." (b) "The area of origin is approximately 40 cm above the floor, with substantial uncertainty." (c) "The spatter shows the defendant struck the victim four times from the right side." (d) "Luminol revealed a latent pattern; we requested confirmatory and DNA testing." (e) "Directionality of the stains is consistent with a source low and to the left."

  2. † An expert testifies to BPA event reconstruction "to a reasonable degree of scientific certainty." Explain why that phrase is especially problematic for event-level BPA conclusions, citing the NAS 2009 finding.

E. Ethics and the human factor

  1. You are a serologist. Your screening test is positive, but you have only enough sample left for one further test: either confirmatory species testing or DNA. The detective is pressuring you to "just run the DNA." How do you reason about which to run, and how do you document the decision?

  2. A BPA analyst is offered a short certification course and a lucrative private-consulting practice testifying for whichever side hires them. Connect this to the NAS report's concern about BPA training and to the "hired gun" problem previewed for Chapter 30.

  3. † Why is communicating the uncertainty of an area-of-origin estimate not merely good manners but a core forensic obligation? Tie your answer to the CSI effect (both directions) and to the duty an expert owes the court rather than the side that called them.

F. Synthesis and cold-case extension

  1. Explain in your own words why the chapter insists that "what is it / whose is it?" and "what happened?" be kept in separate mental boxes. Give one real-world failure that results from merging them.

  2. Rank these blood-related conclusions from most to least defensible, and justify the ranking: (a) "the stain is human blood"; (b) "the spatter is consistent with a low source"; (c) "the DNA excludes the suspect"; (d) "the blood spatter proves a beating occurred here."

  3. † Cold-case extension. The Mill Creek doorframe stain's lower edge lies below the char line, and a low spatter pattern on the same frame is judged inconsistent with a collapse-in-fire. (a) State precisely what these two facts do establish about sequence. (b) State three things they do not establish. (c) Name the next chapter that will test the "before the fire" thread more decisively, and the single autopsy finding (previewed in this book) that would settle whether Diallo was alive when the fire started.

  4. Cold-case extension. Suppose the doorframe stain's DNA (pursued in Chapters 7–9) comes back as a mixture of Diallo and an unknown minor contributor. Using only this chapter and Chapter 9, write the honest one-paragraph status update — what it supports, what it excludes, and the fallacy a detective must not commit when describing the match probability.

  5. Cold-case extension. A volunteer suggests spraying luminol throughout the burned cabin to "find where the murder happened." Give two reasons this is a poor idea in a fire scene specifically, and one circumstance under which limited, well-documented luminol use might still be justified.

  6. Design a one-paragraph protocol for keeping a BPA analyst blind to the case theory in the Mill Creek investigation. What information should they receive, what should they be denied, and at what point in the workflow?