Logical Reasoning · Weaken

PrepTest 116 · Section 2 · Question 20

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

Step 1 · Pre-Question Guidance · read this first

Question type

Weaken

The one idea it tests

If the group you sorted cannot be reliably identified, a link drawn from that sorting falls apart.

The basic logic

The argument finds that one group is more likely to have a trait and concludes the trait causes the group's behavior. Attack the data itself: if nobody can reliably tell which members belong in the group, the comparison is junk.

Tiny words that decide it

“twice as likely”
“conclude”
“causal”

Playground logic

Marcus finds that kids he calls daydreamers carry blue lunchboxes at double the usual rate, so he says blue lunchboxes cause daydreaming. That is a Rooster patty already, but the sharpest pop is this: if nobody can reliably tell a daydreamer from a kid who is just thinking hard, his groups were never real.

Step 2 · The kid story

The Blue Lunchbox Theory

Marcus spent a week watching the class. "Kids who daydream carry blue lunchboxes at double the usual rate!" he declared. "So blue lunchboxes and daydreaming are causally connected." Priya set down her juice box. "How did you decide who was daydreaming?" "Easy. They stared out the window." Nadia raised her hand. "I stare out the window when I do math in my head." Owen added, "And I stare at the wall when I am bored. Did you count me?" Marcus flipped through his notebook. Half the names had question marks. Lulu wrote: if you cannot tell daydreaming apart from other staring, your groups are a guess, and a guess cannot prove a cause. Biscuit, wearing a blue collar, stared out the window too.

Step 3 · Try the model question

Marcus observed that students he classified as daydreamers carried blue lunchboxes at double the rate of other students. He concluded that carrying a blue lunchbox is causally linked to daydreaming.

Which statement, if true, does the most damage to Marcus's reasoning?

Step 4 · The courtroom tale

The Grumpy Customer Study

A bakery owner testified: "Grumpy customers order rye bread at double the usual rate. So rye causes grumpiness. Ban the rye!" Mr. Whitfield rose. "Ma'am, how did you decide who was grumpy?" "They frowned at the counter." "Do people frown when reading a small menu?" "Well, yes." "When the sun is in their eyes?" "Sometimes." Mr. Whitfield turned to the judge. "Your Honor, if nobody can reliably tell grumpy from squinting, her 'grumpy group' is a coin toss. A coin toss proves no cause." The judge kept the rye.

Step 5 · Work the real question

Open PrepTest 116, Section 2, Question 20 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

Spot the structure: one group shows a trait more often, so the trait is said to cause the group's behavior. List ways to pop it: a third factor, reversed cause, or bad data. Then ask whether the group was even identified properly. Prefer the choice that undermines the evidence the whole conclusion rests on. Discard choices about other groups or other behaviors that leave the core comparison standing.