PBS/PBT · September 15 · D07 · Case A STATIC · Period 1-2
The Traces We Leave: compare records and state the limit
One individual D07 trace comparison sheet, Parts A–E.
This page has four parts, in the order you use them. Part 1 is what you need to know. Part 2 says what to do, in plain English. Part 3 shows the work done once on a practice print. Part 4 is your own case and the ten tasks. Everything you write goes on one sheet.
Start: Part 1, then follow the steps in Part 2. Digital deadline: September 15, 11:59 p.m.
Part 1 of 4 · Background and definitions
What you need to know before you write
Everything on today's sheet comes from six short readings. Read R1 to R4 first, and R5 to R6 when you reach the worked example. They stay open while you work: Part A asks you to quote R3 and R4, Part B uses all six with R2, R5 and R6 doing most of the work, Part C uses R3 and R4, Part D uses R1, R3 and R5, and Part E sends you back to R1, R3 and R4 to check your memory.
A useful evidence record preserves what was reported, where it came from and what remains uncertain.
R1. Why comparisons need two sides
A questioned sample has an unknown source. A known sample, also called a reference or elimination sample, has a stated source and is used for comparison. A person's name by itself is not a reference sample. You need comparable features from both sides before deciding whether they are consistent.
Morphology means visible shape and structure. Hair descriptions may include color, thickness, curl, the medulla or central core, and root condition. List the supplied features first. Keeping the features ahead of a person's name helps reduce bias, which is allowing an expectation to influence a judgment.
Used in B1, D1, E3.
R2. A missing input is a real limit
In this packet, the questioned-hair descriptions are supplied, but the known-hair reference features are not. Record the provided features and the missing comparison input. You cannot name a matching or excluded hair source from these descriptions alone. A missing reference is not a mismatch and is not evidence that someone was absent.
Consistent means the compared features agree. Inconsistent means the compared features disagree. Inconclusive means the available information does not support a clear comparison conclusion. For these supplied hair records, explain the input limit instead of guessing a person's name.
Used in B1, B2.
R3. Class features and source conclusions
Class features may be shared by many sources. Evidence showing only these shared features is class evidence. A blood type, hair color or fingerprint pattern family can be a class feature. A sedan is a useful analogy for a broad class: many cars are sedans. A license plate is the analogy for a particular source, but comparing classroom evidence is not as simple as reading a plate.
Individual evidence is the term used when discussing evidence that can support a particular-source conclusion. For fingerprints, detailed ridge features can support a source-identification conclusion when the analysis is sufficient; it does not automatically prove identity. Similar visible hair features can support a possible-source comparison; they do not identify the donor or establish that the person was at the scene.
Used in A1, B1, C1, D1, E1.
R4. What root information can support
Root shape and attached follicular tissue can provide clues about how a hair was removed. Follicular tissue is tissue from around the growing hair that remains attached. A visible root is not the same as attached tissue. These features do not establish when the hair reached the scene.
Attached follicular tissue may provide DNA for testing. Hairs without attached tissue can also contain DNA, but useful results depend on the sample and method. Appearance alone does not provide a DNA result. No DNA test result is supplied in this lesson, and no new test is required.
Used in A2, B1, C1, E2.
R5. The 12-match rule for prints
Minutiae are small fingerprint ridge details, such as endings and splits. Pattern families include loops, whorls and arches. The records state how many details were readable, how many matched a specified card and how many mismatched. Use those supplied counts; do not claim that you counted a missing image.
The 12-match rule, for this classroom exercise only: a model “individual” call needs at least 12 matching minutiae and zero mismatching minutiae. The 12-match rule is a practice rule, not a real forensic identification standard. Under this simplified model, a mismatch gives an inconsistent comparison. With fewer than 12 matches and no mismatches, use an inconclusive call. Keep each call limited to this model.
A match in this classroom comparison supports a possible source under the 12-match rule; it does not by itself prove who touched the object, when, or why. A missing hair reference does not become an “individual” call because another row contains a fingerprint count.
Used in B1, B2, B3, D1.
R6. Report a proportion, not a probability
Matching count divided by readable count, multiplied by 100, gives the percentage of readable features that match. Show the fraction and calculation before rounding. The percentage describes the matching features counted in this exercise. It is not the probability that a person is the source, touched the object or committed an act.
Keep your evidence number beside every case-record statement. Keep the separate worked model and the Part C hypothetical labeled as practice. Neither supplies a missing result for your case.
Used in B1, B3.
Plain-English terms
Fourteen terms, one line each. Part D asks you to use four of them correctly: class evidence, individual evidence, morphology and minutiae. The rest appear in the readings and in your records.
- Questioned sample
- A sample whose source is unknown.
- Known reference
- A sample with a stated source, used for comparison. A name alone is not a reference.
- Morphology
- Visible shape and structure.
- Medulla
- The central core of a hair.
- Bias
- An expectation influencing a judgment.
- Consistent
- The compared features agree.
- Inconsistent
- The compared features disagree.
- Inconclusive
- The available information does not support a clear comparison conclusion.
- Class evidence
- Evidence showing features shared by many sources.
- Individual evidence
- Evidence that can support a particular-source conclusion when the analysis is sufficient.
- Follicular tissue
- Tissue from around the growing hair that remains attached.
- Minutiae
- Small fingerprint ridge details, such as endings and splits.
- Pattern family
- A broad fingerprint pattern, such as a loop, whorl or arch.
- Proportion
- The matching count divided by the readable count in this exercise.
The limit that applies to everything today
These supplied records cannot establish a hair source without known-reference features, or prove who touched an object, when or why. The 12-match rule is a classroom model, not a real forensic identification standard. No DNA result is supplied.
Part 2 of 4 · Directions in plain English
What you will do today, in order
Today you will read trace records, compare the information that is available and explain where a conclusion must stop.
You fill in one sheet with five parts, A to E. The five steps below match those five parts. Each step says what to do, what to use and where to find it on this page.
Step 1. Start in Part 1. Read R1 to R4, the four readings about hair and about what any comparison needs, then answer Part A on your sheet. A1 asks about R3 and A2 asks about R4. With each answer, copy the first three words of the reading sentence you used, so a reader can find it.
Step 2. Now read R5 and R6, the two readings about fingerprints, and then the worked example in Part 3. MODEL-1 shows the whole process on a practice print that belongs to neither case: features first, then the 12-match rule, then the percentage, then the limit. Then go to Part 4 and complete Part B with your own case's records: one row for each hair record, giving its reported features and naming the missing known-reference input, and one row for each of the four print summaries, applying the 12-match rule from R5 to the supplied counts. Finish Part B with B2, one limit you can explain well, and B3, a separate calculation that uses its own numbers, not the model's.
Step 3. Write Part C from the hypothetical facts in the prompt and nothing else. Give three things: the strongest statement those facts support, one stronger statement you may not make and why, and one extra kind of evidence that would be needed and what it would have to show. Do not invent a result, and do not bring in your case's records.
Step 4. Write Part D: one or two sentences about today's records that use all four terms correctly, class evidence, individual evidence, morphology and minutiae. The definitions are in Part 1. A list of definitions on its own does not count.
Step 5. Finish with Part E, then hand in. Answer E1, E2 and E3 from memory first, then check each one against the reading and fix it if you need to. Before you turn in, check that every row carries its evidence number, your case label and its limit. Turn in the one sheet and nothing else.
Use the supplied classroom records for your case. This route is available to everyone. These are not observations you made. You do not need to locate a missing image or sample.
Guided notes and extra help are optional support, not another submission. You may discuss your thinking with a partner, but write your own responses.
The 80-minute plan
- 0–10 min: Open your case handout. Label supplied records, read R1–R4 and answer A1–A2.
- 10–25 min: Follow MODEL-1 and R5–R6. Find the 12-match rule and the percentage method.
- 25–40 min: Complete your own-case hair rows. State the missing reference and needed comparison.
- 40–60 min: Complete four print-summary rows, B2 and B3. Keep model and case numbers separate.
- 60–70 min: Complete C1’s three hypothetical components and D1’s one or two sentences.
- 70–80 min: Complete E1–E3, check evidence numbers and limits, then hand in one sheet.
Working on paper? The complete handout has the same four parts and the response sheet, also as a PDF.
Slides and optional guided notes
The slides follow the same order as this page. Guided notes and extra help are optional support, not another submission. You may discuss your thinking with a partner, but write your own responses.
Download the editable PowerPoint or open it in Google Drive. These slides use a static sequence, with no click animations.
Part 3 of 4 · Worked example
MODEL-1: a finished comparison on a practice print
This example uses a locker print that belongs to neither case. Its counts are supplied; nobody recounted an image. Read it for the order of the work, because Part B asks you to repeat that order with your own case's counts: features first, then the 12-match rule, then the percentage, then the limit.
Begin with the evidence number and source label. Copy the reported features. Check whether the necessary comparison information is actually present. Use the supplied counts or approved reference features only. End with a conclusion no stronger than those inputs support.
The supplied inputs
- Pattern family
- Right loop
- Readable minutiae
- 16
- Card 1
- 13 matching, 0 mismatching
- Card 2
- 5 matching, 3 mismatching
The 12-match rule, from R5: a model-individual call needs at least 12 matching minutiae and zero mismatching minutiae. Any mismatch makes the comparison inconsistent. Fewer than 12 matches with no mismatch is inconclusive. Every print call today uses this one rule.
The five moves, in order
- List the features before considering the card's identity: right loop and 16 readable ridge details. A pattern family is a class feature.
- Compare card 1's supplied count with the 12-match rule: 13 is at least 12, and the record reports zero mismatches. Under this exercise model, this supports the model's “individual” call, consistent with card 1. It is not automatic proof of identity or an action.
- Compare card 2's supplied count with the same model. The record includes three mismatches. Under the 12-match rule, the comparison is inconsistent with card 2. This is an exercise conclusion, not a real forensic exclusion based only on counts.
- Show the card 1 proportion: 13 ÷ 16 × 100 = 81.25%, or about 81% to the nearest whole percent. This percentage describes matching features in the record, not the probability that card 1's source touched the locker.
- Keep the limit beside the comparison. A match in this classroom comparison supports a possible source under the 12-match rule; it does not by itself prove who touched the object, when, or why.
What the finished entry looks like
MODEL-1 reports a right loop with 16 readable minutiae. Card 1 has 13 matches and zero mismatches, meeting the 12-match rule; 13 of 16 is about 81%. Card 2 has five matches and three mismatches, giving an inconsistent comparison under that same model. These practice counts support bounded comparisons, not proof of who touched the locker or when.
A counterfactual, for practice only
The original model also asks you to imagine nine of 16 matching details with zero mismatches. That gives 56.25%, about 56%, and is below the classroom model's 12-match requirement. This counterfactual is practice only. Do not copy its numbers into B3, which supplies its own values.
Do not copy the model's numbers. B3 supplies its own: 14 readable, 9 matching, 0 mismatching. The model's 81% belongs to the model.
Next, Part 4 applies this same order to your case. Open this example on its own page if you want it beside your sheet.
Part 4 of 4 · Your work
Your Case A records and the ten tasks
Everything below is yours to write. First come the 7 records for your case; they are the only inputs for Part B. Then the ten tasks, grouped into Parts A to E. Each task says what to use and where to find it on this page.
4.1 Your Case A records
Supplied classroom simulation records, not observations made by the student. Use only your case. Each item's frame is in your Case A evidence folder: the frame shows where the item was, and the record below says what was reported about it.
Known-hair reference features are not supplied. Record the reported features and the missing comparison input. Do not name a matching or excluded source. A missing reference is not a mismatch or evidence of absence.
E7-H1 hair record
blanket hair
Dark brown; medium thickness; slight wave; continuous medulla; club-shaped root with no tissue.
Known-reference features: not supplied. State this input limit.
E7-H2 hair record
blanket hair
Dark brown; medium thickness; slight wave; continuous medulla; club root, no tissue.
Known-reference features: not supplied. State this input limit.
E7-H3 hair record
blanket hair
Black; coarse; tight curl; fragmented medulla; club root, NO root tissue.
Known-reference features: not supplied. State this input limit.
E8-L1 print summary
fader panel lift
right loop; 14 readable minutiae; 12 matching and 0 mismatching with victim's card.
E8-L2 print summary
fader panel lift
whorl; 8 readable minutiae; 6 matching and 0 mismatching with Cole's card. Reported feature: partial overlay.
E8-L3 print summary
fader panel lift
left loop; 15 readable minutiae; 13 matching and 0 mismatching with cleaner's elimination card.
E8-L4 print summary
can lift
right loop; 13 readable minutiae; 12 matching and 0 mismatching with victim's card.
Print calls use the 12-match rule from R5 and nothing else. A supplied card label is not proof of an action, source or time under a real forensic standard.
4.2 The ten tasks, on one sheet
Write on the response sheet in the handout or on paper. The task letters match the sheet. Extra help sits under each task; it supports that same response and never adds a product.
Part A. Use the reading
Two short answers that quote the reading. This is where you show you read Part 1.
A1 PBS-U1-D07-A1
In R3, which part of the sedan and license-plate analogy represents class evidence, and which represents a particular source? Copy the first three words of the reading sentence you used.
Use: Reading R3 (Part 1)
Need a hint?
Look at the sentence beginning A sedan is, then the next paragraph. An analogy helps explain an idea but does not prove a source.
A2 PBS-U1-D07-A2
According to R4, what clues can root shape and attached tissue provide, and what can they not establish? Copy the first three words of the reading sentence you used.
Use: Reading R4 (Part 1)
Need a hint?
Keep a visible root separate from attached tissue. Describe a clue without turning it into proof of an event.
Part B. Compare your case records
The main work. One row per record in 4.1, then one limit explained in your own words, then one separate calculation. Case A has 7 rows: 3 hair and four prints.
B1 PBS-U1-D07-B1
Complete one comparison row for every supplied questioned item in your case. List its reported features first. For hair, state the missing known-reference input and the comparison you would need; do not name a matching or excluded source. For prints, use the supplied card label and counts to apply the classroom model. Identify class features. Label a model-individual print call only when the 12-match rule allows it; otherwise label the inconclusive or inconsistent comparison. State the limit and keep the evidence number on every row.
Use: Your Case A records (4.1); Reading R1–R6 (Part 1); MODEL-1 (Part 3); F1, the workflow (Part 3)
Need a hint?
Hair: record features, mark known-reference features not supplied, and identify what must be compared. Prints: state matching and mismatching counts before applying the 12-match rule. Missing reference information is not a mismatch.
B2 PBS-U1-D07-B2
Choose your weakest supported comparison or a documented input limit. Give its evidence number and explain exactly what prevents a stronger conclusion. Why is reporting that limit useful?
Use: Your own B1 rows; Your Case A records (4.1); Reading R2,R5 (Part 1)
Need a hint?
Choose from your own case. A missing reference and a low matching count are different limits. Do not claim a specimen is damaged unless the supplied record says so.
B3 PBS-U1-D07-B3
Separate practice: a questioned print has 14 readable minutiae, 9 matching and 0 mismatching. Show the fraction matching and calculate the percentage. What call does our classroom model allow, and what could you honestly say instead of identifying a person?
Use: The numbers in this prompt only; Reading R5,R6 (Part 1); MODEL-1 (Part 3)
Need a hint?
Divide matching by readable, then multiply by 100. Use B3's numbers, not MODEL-1's. Show your calculation before rounding. The percentage is about features, not the probability of a person's involvement.
Part C. A separate hypothetical
Practice writing a bounded conclusion from facts that are not your case.
C1 PBS-U1-D07-C1
Separate hypothetical: suppose a hair is consistent with one known reference and has attached root tissue. Write a three-part practice matrix note: the strongest statement those facts support; a stronger statement you are not allowed to make and why; and one additional type of evidence that could support a more specific source comparison. State what that additional evidence would need to show. No additional test result is supplied or required.
Use: Hypothetical facts in this prompt; Reading R3,R4 (Part 1)
Need a hint?
Keep this hypothetical separate from your case and any image. The reading explains the limits of visible similarity and that a DNA result would have to be obtained and interpreted, not inferred from appearance.
Part D. Use the vocabulary
One or two sentences that use four terms from Part 1 correctly.
D1 PBS-U1-D07-D1
Use class evidence, individual evidence, morphology and minutiae correctly in one or two sentences about today's records. A list of definitions alone does not count.
Use: Reading R1,R3,R5 (Part 1); Your own B1 rows
Need a hint?
Connect the four terms to the comparison work. Keep individual calls qualified as classroom-model conclusions.
Part E. Check yourself, then check the reading
Three one-line answers from memory, then fix them against Part 1. Checking is part of the work, not a second submission.
E1 PBS-U1-D07-E1
Blood type B at a scene: class or individual evidence? Answer in one line from memory, then check R3.
Use: Reading R3 (Part 1)
Need a hint?
Ask whether the feature can be shared by many sources.
E2 PBS-U1-D07-E2
What clues can root features provide, and what can they not establish? Answer in one line from memory, then check R4.
Use: Reading R4 (Part 1)
Need a hint?
A clue about removal is not a time of arrival.
E3 PBS-U1-D07-E3
Why list features before considering a person's name? Answer in one line from memory, then check R1.
Use: Reading R1 (Part 1)
Need a hint?
Use the idea of bias in your explanation.
Turn in one sheet
Complete your D07 trace comparison sheet during class. Hand the paper sheet to me before leaving, or submit one PDF to this Schoology assignment by Tuesday, September 15, at 11:59 p.m. Submit once. Keep notebook pages in your notebook.
Turn in only the completed Parts A–E sheet. No extra CER, tracker, specimen image, microscope drawing or notebook upload is required.
Case A: submit to the D07 assignmentKeep your source-labeled comparison and its limits for the next class on Thursday, September 17. Your notebook pages stay in your notebook.
Help when you need it
Need help? Choose a starting point
Restart where you stopped
Find your next unanswered task ID. For hair, use R1–R4. For prints and percentages, use R5–R6 and MODEL-1. Check that each conclusion includes its evidence number and limit.
Notebook support, not another submission
Keep notebook pages in your notebook. They are not an additional submission.
Matrix headings: Evidence number / Supplied record statement / What this supports / Limit or missing input.
- What information is supplied?
- What reference is available or missing?
- What can the comparison support?
- What can it not support?
Write a brief reminder of how to keep a comparison within its evidence.
Optional references for a separate lesson
Additional simulations for a separate teacher-directed lesson. These are not required for today's trace comparison sheet.
This assignment reviews supplied records. It does not require handling specimens, collecting material or running a test. Practical work needs separate teacher direction and approved preparation.
Your teacher may offer an actual classroom-record route only when complete, approved questioned and reference inputs are available. Follow the preparation requirements for that route. Finishing this record exercise does not give a practical-lab pass or mark an official PLTW activity complete.
Connection to PLTW
Related PLTW reference:
- Activity 1.1.3 The Traces We Leave Behind
In Lesson 1.1, Investigating the Scene. This John Hay classroom adaptation practices describing evidence, comparing available information and explaining limits.
This sheet does not replace or certify completion of the official activity. If assigned, open the matching activity through your existing myPLTW account or section hub.
Open your official PLTW section hubSources and optional background
The required explanation is supplied above. These links do not add an assignment.
- S1. FBI Handbook of Forensic Services
limits of hair comparison.
- S2. NIST: follicular tag
attached follicular tissue.
- S3. NIJ-supported research on rootless hair DNA
why appearance alone cannot decide DNA-test usefulness.
- S4. Department of Justice: latent print testimony and reporting
limits on source conclusions.
- S5. NIJ Fingerprint Sourcebook
fingerprint comparison background. The 12-match rule above is a teaching model.
These supplied records cannot establish a hair source without known-reference features, or prove who touched an object, when or why. The 12-match rule is a classroom model, not a real forensic identification standard. No DNA result is supplied.