LSAT Parallel Reasoning and Parallel Flaw: Match the Skeleton

Parallel Reasoning questions are some of the longest in Logical Reasoning: six full arguments to read for one point. Students often fall behind on these, because they read every answer in full and compare everything to everything. There's a faster way. This guide shows how to strip an argument to its skeleton, which parts to match first, and how Parallel Flaw differs.

1What these questions ask

• Parallel Reasoning: "Which one of the following arguments is most similar in its reasoning to the argument above?"
• Parallel Flaw: "Which one of the following arguments exhibits flawed reasoning most similar to that in the argument above?"

In both, you're matching the shape of the argument. The topic doesn't matter. An argument about dolphins can be perfectly parallel to one about tax law.

2Step 1: Strip the argument to its skeleton

Replace the topic with letters and keep only the logic. For example:

"Every senior on the debate team has won a regional title. Marcus is a senior on the debate team. So Marcus has won a regional title."

Skeleton: All A are B. X is A. So X is B.

Things that matter in the skeleton:

• Certainty words: all, most, some, must, probably, likely, should.
• Conditional and causal language: if, only if, causes, leads to.
• The type of conclusion: a fact, a prediction, a recommendation, a comparison, a causal claim.

Things that don't matter: the subject, the order the sentences appear in, and the specific nouns.

3Step 2: Match the conclusion first

This is the step that saves time. Before comparing whole arguments, compare only the conclusions.

Ask two questions about the original conclusion:

1. How certain is it? "Must," "probably" and "should" are different conclusions.
2. What form is it? A conditional ("if X then Y"), a definite fact ("X is Y"), a recommendation ("X should do Y"), a negative ("X is not Y").

Cross out every answer whose conclusion doesn't match. On many questions that removes two or three answers before you've read a single premise closely.

4Step 3: Match the premises

For the survivors, check the premises. Do they have the same certainty (all vs. most vs. some)? The same kind of link (conditional vs. causal)? Remember that order doesn't matter. The original can state the conclusion first and the right answer can state it last.

5Parallel Flaw: name the flaw first

Parallel Flaw adds one step: before you read the answers, name the flaw in plain words. The right answer must make the same mistake, and only that mistake.

Common flaws in these questions:

• Reversal: If A then B; B; so A.
• Negation: If A then B; not A; so not B.
• Some + some: Some A are B; some B are C; so some A are C.
• Causal jump: A happened with B, so A caused B.

If you can't name the flaw yet, check our guide to flaw questions and the "takes for granted" guide, which cover the vocabulary these answers use.

A trap specific to Parallel Flaw: an answer with a different flaw. It's flawed, so it feels right. But it has to be flawed in the same way.

6Sample question (LSAT JET original)

If the orchard gets a late frost, its apple harvest will be small. This year the orchard's apple harvest was small. So the orchard must have gotten a late frost.

Which one of the following arguments exhibits flawed reasoning most similar to the reasoning above?

(A) If a car is serviced on schedule, it will pass inspection. Dana's car passed inspection. So Dana's car must have been serviced on schedule. (B) If a store raises prices, it will lose customers. The store did not raise prices. So it will not lose customers. (C) If a student studies daily, she will improve. Lena studied daily. So Lena will improve. (D) If the bridge is closed, traffic will be heavy. Traffic is not heavy. So the bridge must not be closed. (E) Some gardeners use compost, and some compost users grow tomatoes. So some gardeners must grow tomatoes.

Answer: (A).

The original: If A then B. B. So A. That's a reversal. A small harvest could have other causes.

• (A) If serviced then passes; passed; so serviced. Same reversal, same "must." Match.
• (B) If A then B; not A; so not B. A flaw, but a different one (negation).
• (C) If A then B; A; so B. Valid. Not flawed at all.
• (D) If A then B; not B; so not A. The contrapositive. Valid.
• (E) A "some + some" flaw. Wrong shape entirely.

Notice how fast the conclusion-first check works here: (C) concludes a prediction, and (D) is valid, so they go as soon as you see the skeleton.

7When two answers survive

Sometimes two answers match the conclusion and the premises look close in both. Go back to the details you skipped:

• Quantity: "most" in the original needs "most" in the answer, not "some."
• Negatives: a premise that says "no A are B" isn't matched by "some A are not B."
• Number of premises: an argument with two premises usually isn't parallel to one with three.

One of these will almost always split the last two.

8Timing these questions

Parallel questions often take two to three minutes even with a good method. Many of my students flag them on the first pass and come back after the shorter questions. Points are worth the same everywhere, so spend your minutes where they're cheapest. Our timing strategy guide explains how to set that up.

9How to practice

• Pull five Parallel and five Parallel Flaw questions.
• For each, write the skeleton in letters before reading the answers.
• For Parallel Flaw, also write the flaw's name.
• Then cross out answers by conclusion only, and note how many you removed. The number should climb with practice.