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Logical Reasoning · Parallel Flaw

PrepTest 113 · Section 4 · Question 4

Phase 1 (your first ~400 questions): read the guidance, then answer. No clock.

Step 1 · Pre-Question Guidance · read this first

Question type
Parallel Flaw
The one idea it tests
Most people in a group share a trait; this person has the trait; so this person must be in the group. That is the Backwards Arrow.
The basic logic
Use the X-Ray Method. Strip the subject down to letters: 'Many X are T. This person is T. So this person must be X.' The flaw reverses the direction: being in the group tends to come with the trait, but having the trait does not put you in the group. Find the choice with the same skeleton and the same certainty word.
Tiny words that decide it
  • “many”
  • “must be”
  • “therefore”

Playground logic

Most spelling bee winners read every night, and Nadia reads every night, so Marcus declares she must be a winner. The arrow runs from winners to reading, and Marcus drives it backwards, from reading to winners.

Step 2 · The kid story

Marcus Crowns a Champion

Marcus looked at the spelling bee trophy case. 'I've done research,' he announced. 'Almost every spelling bee champion at our school reads every single night.' He pointed across the library. 'Nadia reads every single night. So Nadia must be a spelling bee champion.' Nadia looked up from her book. 'I've never even entered.' Priya sipped her juice box. 'Marcus, lots of kids read every night. Owen does. Lulu does. Even Kevin reads dinosaur books every night.' Kevin nodded proudly. Lulu drew two circles on her clipboard, a small one for champions and a giant one for nightly readers. 'Champions are mostly inside the readers circle,' she said. 'But being a reader doesn't put you inside the champions circle.' Marcus stared at the circles. 'This drawing is very unfair to my theory.'

Step 3 · Try the model question

Most of our school's spelling bee champions read every night. Nadia reads every night. So Nadia must be a spelling bee champion.

Which argument makes the same reasoning mistake as the one above?

Choices

Step 4 · The courtroom tale

The Case of the Muddy Boots

A prosecutor pointed at the defendant. 'Most of the people who trampled Mrs. Pell's garden wore muddy boots. The defendant wore muddy boots that day. Therefore he must be one of the tramplers.' Mr. Whitfield rose slowly, thumbs in his suspenders. 'Your Honor, it rained all morning. Half this town wore muddy boots, including, I notice, the prosecutor.' The prosecutor glanced at his feet. Mr. Whitfield continued. 'Tramplers tend to have muddy boots. That does not make every muddy-booted person a trampler. My learned friend has run the arrow backwards.' Judge Rowan Pike sighed. 'Unless there is more than mud, the charge cannot stand.'

Step 5 · Work the real question

Open PrepTest 113, Section 4, Question 4 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

Write the original as letters: a group, a trait most of them share, one person with the trait, and the 'must be' conclusion putting that person in the group. Then X-ray each choice into letters. Keep only the one where the person has the trait and is concluded to be in the group, with the same strong certainty. Toss choices that run the arrow forwards, that conclude about a whole group, or that slip in a cause.