PrepTest 152 · Section 2 · Question 16 (Parallel Flaw)

Step 1 · Pre-Question Guidance · read this first

Question type

Parallel Flaw

The one idea it tests

Knowing that most people who do something have a trait does not tell you that someone with the trait will do that thing.

The basic logic

X-Ray the bones: 'Most who do M have trait P. So a certain person will likely do M if they get P.' That is a Backwards Arrow: the evidence runs from M to P, but the conclusion runs from P to M. Find the choice with the same reversal, the same 'likely,' and the same 'if' at the end.

Tiny words that decide it

“most”
“likely”
“if”
“therefore”

Playground logic

Most kids who visit the nurse's office have a scraped knee. Nadia, a student at the school, concludes Owen will likely go to the nurse if he scrapes his knee. The arrow got flipped: lots of scraped kids may never go.

Step 2 · The kid story

Nadia's Nurse Prediction

Nadia helps in the school office. She notices something. 'Most kids who come to the nurse's office have a scraped knee!' she tells Owen. 'So, since you go to this school, you'll probably go to the nurse if you scrape your knee.' Owen laughs. 'I scraped both knees at recess yesterday. I just put a sticker on them and kept playing.' Priya sips her juice box. 'Nadia, you only counted kids who walked in. You never counted all the kids with scraped knees who stayed outside.' Nadia looks at her chart. 'Oh. Most nurse visitors have scrapes. That's not the same as most scraped kids visiting the nurse.' Lulu draws an arrow on her clipboard, then flips it. 'Backwards Arrow!' she says.

Step 3 · Try the model question

Most kids who visit the nurse's office have scraped knees. Owen goes to this school. So it is likely that Owen will visit the nurse's office if he scrapes his knee.

Which argument has a flaw most like the flaw in Nadia's argument?

Step 4 · The courtroom tale

The Case of the Umbrella Prediction

In a fictional town, a forecaster sues a bus company for missing riders. 'Your Honor,' she says, 'most people who ride the bus carry umbrellas. Ms. Lane lives on the bus route. So she'll likely ride the bus if she buys an umbrella. The company should have saved her a seat.' Mr. Whitfield nearly drops his suspenders. 'Ma'am, most bus riders have umbrellas. That does not mean most umbrella owners ride the bus. Thousands walk, drive or bike with umbrellas.' The judge bangs the gavel. 'Case dismissed. You flipped the arrow.' The forecaster sheepishly closes her umbrella.

Step 5 · Work the real question

Open PrepTest 152, Section 2, Question 16 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

X-Ray the stimulus first: write its bones with letters. Note the quantity word, the direction of the evidence, and the direction of the conclusion. Confirm it is flawed and name the flaw. Then X-Ray each choice. Throw out any whose conclusion runs in the same direction as its evidence (no flaw), any that adds a negation, and any that changes the condition at the end. Keep the one whose bones match, flaw included.