Logical Reasoning · Weaken

PrepTest 134 · Section 1 · Question 13

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

Two groups end up with the same result, but the groups were not equal to start with, so 'same result' can hide a real benefit.

The basic logic

The argument compares people who do a helpful habit with people who don't, finds the same outcome, and concludes the habit does nothing. The secret sauce is that the groups started out alike. A great weakener shows that the people more at risk were the ones more likely to pick up the habit, so the habit may be pulling them back up to even.

Tiny words that decide it

“roughly the same”
“indicates”
“does not”

Playground logic

Marcus sees that kids with knee pads and kids without knee pads end up with equally many scrapes, so he says pads are useless. Priya points out that the wobbliest riders are the ones who wear pads. If wobbly riders with pads only scrape as much as steady riders without them, the pads are doing real work.

Step 2 · The kid story

Useless Knee Pads?

Marcus counted scrapes at the scooter park all week. Ten kids wore knee pads. Ten kids didn't. Both groups ended up with about equally many scrapes. "Case closed," Marcus announced. "Knee pads don't prevent scrapes." Priya looked at the pad group. There was Kevin, who falls off everything. There was Tommy, who steers with his elbows. There was Lulu, who just learned to ride last Tuesday. "Marcus," Priya said, "who wears pads?" "The kids who fall a lot, I guess. Their parents make them." "So the pad group is full of the wobbliest riders," Priya said, "and they still only got as many scrapes as the steady kids. That sounds like the pads are helping a lot." Marcus took off his detective hat. "Huh. My groups weren't fair."

Step 3 · Try the model question

Marcus: At the scooter park, I compared kids who wear knee pads with just as many kids who don't. Both groups ended up with about equally many scrapes. This shows that knee pads do not help prevent scrapes.

Which fact, if true, would hurt Marcus's argument the most?

Step 4 · The courtroom tale

The Helmet Lawsuit

A helmet maker was being sued. "Riders with this helmet and riders without it had equally many bumps," said the plaintiff's lawyer, Ms. Vance. "The helmet does nothing!" Mr. Whitfield stood up slowly. "Your Honor, who buys this helmet?" He called a shop owner. "Mostly beginners," she said. "The shaky ones. Experts rarely bother." Mr. Whitfield turned to the jury. "So shaky beginners wearing the helmet got no more bumps than steady experts without it. That is not proof of a useless helmet. That is a helmet working hard." Judge Cole raised an eyebrow at Ms. Vance. "Your comparison groups seem rather lopsided, Counselor."

Step 5 · Work the real question

Open PrepTest 134, Section 1, Question 13 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

Spot the setup: two groups, one with a habit and one without, same outcome, so the habit is said to do nothing. Ask whether the groups started out equal. Look for the choice that shows people more likely to have the bad outcome were also more likely to have the habit. That hides a real benefit. Skip choices that compare to outside rates, that support the 'no effect' story, or that talk about different activities.