Valid or Fallacious? Auditing a Detective's Reasoning
- Grade band
- 9-10
- Duration
- 50 minutes
- Skill
- Identifying fallacious reasoning in an argument
- Standards
- RI.9-10.8 — Delineate and evaluate the argument and specific claims in a text, assessing whether the reasoning is valid and the evidence is relevant and sufficient; identify false statements and fallacious reasoning.
- MP.3 — Construct viable arguments and critique the reasoning of others.
- Materials
- Detective's case report and student worksheet (printable, included)
- Reasoning sort cards (printable, included)
- Highlighters
Learning objectives
- Students will be able to delineate the claims in an argument and identify which claims the conclusion depends on.
- Students will be able to identify and name fallacious reasoning — hasty generalization, false cause, false dilemma, ad hominem, and affirming the consequent — and justify each label.
- Students will be able to evaluate whether evidence is relevant and sufficient, and revise a flawed conclusion so it follows from the evidence.
Lesson procedure
- Hook (5 minutes) — I Do. Read the final line of the case report aloud: “Conclusion: Mei stole the cash box.” Ask for a quick vote — convinced or not? Explain that today students will audit the detective's work the way an editor or a judge would: not “is the conclusion true?” but “does the reasoning get us there?”
- Fallacy toolkit (10 minutes) — I Do. Walk through the reference table in Part 1 with one everyday example of each. Emphasize the difference between a false statement (a factual claim that's wrong) and fallacious reasoning (a step that doesn't follow, even if every statement is true).
- Practice sort (8 minutes) — We Do. Students sort the eight practice cards in pairs (or use the smartboard sort). For each fallacy card, partners must say what additional evidence would be needed to make the claim reasonable.
- Audit the report (15 minutes) — You Do. Students label each numbered claim in the report as Valid/Sound or name its fallacy, writing a one-sentence justification in the audit table.
- Rewrite (10 minutes) — You Do. Students complete Part 3: identify which claims the detective's conclusion depends on, and write a revised conclusion that the evidence actually supports.
- Debrief (2 minutes) — Closure. Ask one or two students to read their revised conclusions. Note how often the honest conclusion is narrower — and more interesting — than the original.
Printable worksheet
Part 1: Fallacy reference.
| Fallacy | What goes wrong |
|---|---|
| Hasty generalization | A broad conclusion is drawn from too few or unrepresentative examples. |
| False cause | Because one thing happened before or alongside another, it's assumed to have caused it. |
| False dilemma | Only two options are presented when others exist. |
| Ad hominem | A claim is rejected because of something about the person, not the claim itself. |
| Affirming the consequent | “If P then Q. Q is true. So P is true.” Q may have other causes. |
Part 2: Audit the case report.
CASE REPORT — The Bake Sale Cash Box Filed by Detective R. Lark
[1] The cash box was last seen on the bake sale table at 11:00 and was missing at 11:20.
[2] Twice this year, students working the bake sale table were late returning supplies. Bake sale volunteers clearly cannot be trusted with money.
[3] Jonah started volunteering last month, and since then the sale has come up short twice. Jonah's arrival is what's causing the missing money.
[4] The security camera, which shows the table the whole time, records only Mei and Harper at the table between 11:00 and 11:20. Therefore Jonah did not take the box from the table.
[5] So either Mei took the box or Harper did. There is no other possibility.
[6] Harper's older brother was suspended last year, so Harper's statement that she didn't touch the box shouldn't be believed.
[7] The cash box was sticky with frosting, so whoever took it would have frosting on their hands. Mei has frosting on her hands. So Mei took the box.
[8] The office log shows a cash box was turned in at 11:12, and the bake sale box is the only cash box at school today.
Conclusion: Mei stole the cash box.
| Claim | Valid / sound, or which fallacy? | Why (one sentence) |
|---|---|---|
| [2] | ||
| [3] | ||
| [4] | ||
| [5] | ||
| [6] | ||
| [7] |
Part 3: Evaluate and rewrite.
1. Which numbered claims does the detective's conclusion actually depend on? Are they valid, and is the evidence sufficient?
2. Which claim did the detective ignore, and how does it change the case?
3. Rewrite the conclusion so that it follows from the evidence. Cite the claims you rely on.
Part 4: Practice sort cards. Cut apart and sort into Valid, Hasty generalization, False cause, and False dilemma.
Try it on your smartboard
The same practice sort, on screen. Drag each argument to the right category — works with touch, a stylus, or a mouse.
Valid
Hasty generalization
False cause
False dilemma
Background for the teacher
A valid argument is one where the conclusion must be true if the premises are; a sound argument is valid and has true premises. Fallacies are recurring patterns of invalid or unsupported reasoning, and naming them is useful mainly because it forces students to say precisely where an argument breaks. Students should always be able to explain the label in their own words — “that's a false dilemma” is the beginning of an answer, not the answer.
The case report is built so that its conclusion leans on two flawed claims: [5], a false dilemma (the box could have been moved by someone with a legitimate reason), and [7], affirming the consequent (Mei may have frosting on her hands simply because she was working a bake sale). Claim [4] is genuinely valid, given its premise that the camera covers the table the whole time. Claim [8] is the evidence the detective never uses: if the only cash box at school was turned in to the office at 11:12, the most supported conclusion is that the box was moved, not stolen. That's the lesson's larger point — fallacies often hide the most relevant evidence in plain sight.
Differentiation
Support: Pre-label claims [2] and [3] for students, and give a word bank of fallacy names for the rest. Allow students to audit only claims [4]–[7].
Extension: Have students find a real editorial, ad, or social-media post and audit it with the same table, distinguishing false statements from fallacious reasoning.
ELL support: Provide each fallacy name with a simple paraphrase (“too few examples,” “before ≠ because,” “only two choices,” “attack the person,” “backward if–then”) and an icon, and pre-teach conclusion, premise, and evidence.
IEP/504 support: Reduce the practice sort to four cards (one per category), allow the audit table to be completed with fallacy names only (justifications given orally), and provide the Part 3 rewrite as a sentence frame: “The evidence supports ___ because ___.”
Answer key (for teachers)
Part 2 — Audit. [2] Hasty generalization — two late returns don't support a claim about all volunteers or about money. [3] False cause — the shortfalls followed Jonah's arrival, but nothing links him to them. [4] Valid — if the camera shows the table continuously and he isn't on it, he didn't take it from the table. [5] False dilemma — the box could have been moved by someone else or for a legitimate reason (see [8]). [6] Ad hominem — a sibling's record says nothing about Harper's statement. [7] Affirming the consequent — frosting on Mei's hands is expected for a bake sale worker and doesn't show she took the box.
Part 3. 1. The conclusion depends on [5] and [7], both fallacious; the evidence is insufficient. 2. Claim [8] — the box was most likely turned in to the office at 11:12, which suggests it was moved rather than stolen. 3. Sample: “Based on [1], [4], and [8], the cash box was most likely turned in to the office at 11:12; there is not enough evidence that anyone stole it.” Accept reasoned alternatives that cite the claims accurately.
Part 4 — Sort. Valid: peanuts/Rhea; key out of town. Hasty generalization: witnesses; exact change. False cause: lights flickered; table moved. False dilemma: catch the thief today; agree or protect the thief.