Logical Reasoning · Flaw

PrepTest 133 · Section 2 · Question 6

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

Step 1 · Pre-Question Guidance · read this first

Question type

Flaw

The one idea it tests

Evidence about a few members of a group can't prove a claim about every member of that group.

The basic logic

The evidence shows that some members of a group existed before their supposed source, so those particular ones couldn't have come from it. The conclusion leaps to 'none of the group came from that source.' That is a Scope Shift from some to all.

Tiny words that decide it

“some”
“all”
“none”

Playground logic

Marcus's patty: a few paper cranes were folded before the folding book arrived. His top bun: no crane in the class came from that book. The patty only covers those few cranes, but the top bun covers every crane, which is a classic Scope Shift.

Step 2 · The kid story

Detective Marcus and the Paper Cranes

Ms. Rivera's class had a shelf full of paper cranes. Everyone said the cranes came from the origami book that arrived on Tuesday. Marcus pulled out his magnifying glass. "Aha! This crane is dated Monday. And so is this one. They were folded before the book even got here!" He spun around. "So none of these cranes came from that book. Case closed." Priya sipped her juice box. "You found two cranes from Monday. There are forty cranes on that shelf." "So?" "So those two couldn't come from the book. But the other thirty-eight could have." Marcus lowered his magnifying glass. Lulu wrote on her clipboard: Two early cranes do not make all cranes early.

Step 3 · Try the model question

Marcus: Everyone says the class's paper cranes were made using the origami book that arrived Tuesday. But I found cranes dated Monday, before the book arrived. So these cranes prove that no crane on the shelf was made using that book.

Which choice best describes the mistake in Marcus's reasoning?

Step 4 · The courtroom tale

The Quilt Square Dispute

A quilting club claimed all its quilt squares were cut with a new pattern kit. A rival club's lawyer stood up. "Your Honor, here are three squares sewn last spring, before the kit was ever sold. Therefore no square in this club was made with the kit!" Mr. Whitfield rose slowly. "Counsel has shown that three squares predate the kit. The club owns three hundred squares. Three early squares tell us about three squares. They tell us nothing about the other two hundred ninety-seven." The judge peered over her glasses. "Evidence about a few is not evidence about all. Objection sustained."

Step 5 · Work the real question

Open PrepTest 133, Section 2, Question 6 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

Find the evidence and ask exactly which members of the group it covers. Then find the conclusion and ask how many members it covers. If the evidence covers some and the conclusion covers all (or none), you have a Scope Shift. Choose the answer that names an overly broad generalization. Before picking anything else, check whether it describes a move the argument actually makes.