Logical Reasoning · Flaw

PrepTest 151 · Section 3 · Question 18

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

Step 1 · Pre-Question Guidance · read this first

Question type

Flaw (overlooks a possibility)

The one idea it tests

A pattern in a crowd over time is not proof that each single person follows that pattern, because the crowd itself may change.

The basic logic

The evidence is an average taken at different times. The conclusion is about how one typical person changes. Ask: are the same people being counted at each time? If different people show up at different times, the shift could come from who is counted, not from anyone changing.

Tiny words that decide it

“on average”
“typically”
“a person”

Playground logic

Marcus sees big books checked out in the morning and skinny books in the afternoon, and decides every kid starts big and switches to skinny. His patty is a crowd average; his top bun is about one kid's day. The secret sauce he never checks: the morning crowd and the afternoon crowd are the same kids.

Step 2 · The kid story

The Library Switcheroo

Marcus taps the library sign-out sheet with his pencil. "Case closed," he says. "In the morning, kids check out giant chapter books. By afternoon, it's all skinny picture books. So every kid starts the day brave and gets tired by lunch." Priya sips her juice box and flips the sheet over. "Who signs out in the morning?" she asks. Marcus squints. The fifth graders have library time at nine. The second graders come at two. "Oh," says Marcus. Lulu writes on her clipboard: Different kids, different times. Nobody switched anything. The big books and small books were picked by two different crowds. Marcus wanted a story about one kid's day, but his sheet only told him which crowd walked in at which hour. Biscuit, who was not allowed in the library, barked anyway.

Step 3 · Try the model question

Library records show that big chapter books are checked out mostly in the morning, while skinny picture books are checked out mostly in the afternoon. Marcus concludes that a typical student reads big books early in the day and switches to small books later.

Marcus's reasoning is open to criticism because it never considers that

Step 4 · The courtroom tale

The Case of the Changing Cones

Mr. Whitfield snapped his suspenders. "Your Honor, the ice cream truck sells vanilla before noon and chocolate after. My opponent claims every customer starts with vanilla and turns to chocolate." He held up a schedule. "But the morning customers are the walkers from the retirement home. The afternoon customers are the kids out of summer camp. No single person changed flavors at all." The judge nodded. "So the pattern comes from who shows up, not from anyone changing their mind." "Exactly," said Mr. Whitfield. "Two crowds, one truck, zero switching." The opposing lawyer quietly put down his spoon.

Step 5 · Work the real question

Open PrepTest 151, Section 3, Question 18 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

Find the evidence: a group average measured at several times. Find the conclusion: a claim about how one typical person changes over those times. Ask whether the same people are being counted at each time. Look for the answer that says the group at one time could be made of different people than the group at another time. Skip answers that bring in new time periods, other ways of measuring, or people who are not counted at all.