Logical Reasoning Skills: How to Test, Boost, and Undermine Arguments

Strengthen the Argument

What “strengthen” means on the LSAT

To strengthen an argument means to make the conclusion more likely to be true given the premises. On LSAT Logical Reasoning, you’re not trying to prove the conclusion beyond doubt. You’re looking for an answer choice that adds support—often by shoring up a weak link the argument depends on.

A useful way to picture most LR arguments is as a bridge:

• The premises are the supports you’re given.
• The conclusion is where the author wants you to go.
• The missing plank is the assumption—something the author is taking for granted.

Strengthen questions reward you for finding that missing plank and adding material that makes the gap smaller.

Why strengthening matters (and what the test is really measuring)

Strengthen questions test whether you can:

• Distinguish what is stated (premises) from what is claimed (conclusion).
• Notice what would have to be true for the reasoning to work (assumptions).
• Predict what kind of new information would increase confidence in the conclusion.

In real life, this is the skill of evaluating persuasive claims—ads, political arguments, workplace proposals—by asking, “What would make this claim more credible?”

How strengthening works: the mechanism

Most LSAT arguments are vulnerable in predictable ways. Strengthening typically works by doing one (or more) of the following:

1. Confirming an assumption

   • If the argument relies on an unstated idea, a strong strengthener often states that idea (or something very close).

2. Eliminating alternative explanations

   • Especially in causal arguments (“A caused B”), strengthen answers often rule out other plausible causes.

3. Adding relevant evidence

   • New data that directly supports the conclusion can strengthen—particularly when it connects the premise more tightly to the conclusion.

4. Showing the reasoning pattern is reliable

   • In arguments by analogy or generalization, strengthening can show the analogy is appropriate or the sample is representative.

5. Clarifying definitions or resolving ambiguity

   • Sometimes the argument trades on a key term (“effective,” “safe,” “significant”). Strengthening can lock down the intended meaning.

A critical mindset shift: strengthening is about relative improvement, not perfection. Some answer choices might sound helpful but don’t actually touch the argument’s vulnerability.

A step-by-step approach you can apply consistently

1. Identify the conclusion (What is the author trying to prove?)
2. Identify the premises (What reasons are offered?)
3. Describe the gap in your own words
   • Ask: “Why might this conclusion not follow?”
4. Predict what would help
   • Not the exact answer—just the type (rule out an alternative cause, confirm the sample, etc.).
5. Test answer choices for relevance to the gap
   • The right answer increases likelihood of the conclusion.
   • Wrong answers often: restate premises, are out of scope, strengthen a different claim, or add irrelevant details.

Common strengthening “families” (with what they look like)

Causal conclusions

If the conclusion says one thing caused another, typical strengthen moves include:

• Showing the cause happened before the effect.
• Showing the effect doesn’t happen without the cause.
• Ruling out a third factor.
• Showing a plausible causal mechanism.

Generalizations from samples

If the conclusion generalizes from a group studied to a broader group, strengthen moves include:

• Showing the sample is representative.
• Increasing sample size or quality.
• Showing similar results across different populations.

Plans/proposals

If the conclusion recommends a plan (“We should do X”), strengthen moves include:

• Showing X will achieve the goal.
• Showing X is feasible (cost, time, compliance).
• Showing alternatives are worse.

Strengthen in action (Example 1: Causation)

Stimulus: “After the city installed LED streetlights, nighttime car accidents decreased. Therefore, the new LED streetlights caused the decrease in nighttime accidents.”

Gap: The argument assumes nothing else changed that could explain the decrease (traffic enforcement, speed limits, road redesign, weather patterns, etc.). It also assumes the timing isn’t coincidence.

Correct strengthen idea: Rule out a major alternative cause or show a direct link between LEDs and fewer accidents.

Answer choice (strengthener): “During the period after the LED streetlights were installed, there were no other changes to traffic laws, enforcement levels, or road design that would be expected to reduce nighttime accidents.”

Why it strengthens: It removes competing explanations, making the causal claim more plausible.

Tempting wrong choice: “Residents report that the LED lights make streets look more modern.”

• Sounds positive but doesn’t address accidents or causation.

Strengthen in action (Example 2: Generalization)

Stimulus: “In a survey of employees at Company A, most said they prefer working from home. So, most employees prefer working from home.”

Gap: Company A employees may not represent employees generally.

Answer choice (strengthener): “Company A’s workforce is similar to the national workforce in age, job type, and geographic distribution.”

Why it strengthens: It supports representativeness, making the leap from the sample to the population more justified.

Exam Focus

• Typical question patterns:
  • “Which of the following, if true, most strengthens the argument?”
  • “Which of the following, if assumed, would most support the conclusion?”
  • “Which of the following provides the most support for the claim above?”
• Common mistakes:
  • Choosing an answer that is merely consistent with the conclusion rather than making it more likely.
  • Falling for answers that strengthen a premise or a side point but don’t connect to the conclusion.
  • Ignoring the argument’s type (causal, sampling, plan) and therefore looking for the wrong kind of support.

Weaken the Argument

What “weaken” means on the LSAT

To weaken an argument means to make the conclusion less likely to be true given the premises. Like strengthening, weakening is about probability and support—not about conclusively disproving the conclusion.

A weaken answer typically attacks the argument at its most vulnerable point. If strengthening adds a plank to the bridge, weakening removes one or reveals the bridge was never connected.

Why weakening matters

Weaken questions measure your ability to think like a critical reader: to spot the difference between “this sounds plausible” and “this is actually supported.” In real-world reasoning, this is how you avoid being misled by impressive-sounding claims that ignore alternatives, confuse correlation with causation, or overgeneralize from limited evidence.

How weakening works: the mechanism

Weakening usually works by doing one (or more) of these:

1. Providing an alternative explanation

   • The classic move against causal arguments: “Something else caused the effect.”

2. Showing the cause is present without the effect (or vice versa)

   • If A supposedly causes B, evidence that A happens but B doesn’t (or B happens without A) weakens.

3. Attacking representativeness or methodology

   • Show the sample is biased, too small, self-selected, or measured incorrectly.

4. Introducing a counterexample

   • If the conclusion is a broad generalization, even one credible counterexample can weaken (depending on how strong the generalization is).

5. Showing the plan won’t work or has hidden costs

   • For recommendation arguments, show it won’t achieve the goal or creates a worse problem.

One subtlety: weakening does not require the new information to be certainly true in reality—only that if true, it would undermine the support.

A step-by-step approach

1. Find the conclusion
2. Find the support
3. Name the assumption (what must be true for the reasoning to work)
4. Negate or challenge that assumption
   • A great weaken answer often looks like “What if that assumption is false?”
5. Check impact
   • The correct answer should make you think, “Oh—that could explain things without the conclusion,” or “Then the reasoning doesn’t hold.”

Common weakening “families” (and how they trick you)

Causal arguments: correlation vs. causation

LSAT loves causal reasoning because it has predictable vulnerabilities:

• Reverse causation: Maybe B causes A.
• Third-variable causation: C causes both A and B.
• Coincidence/timing: The relationship is accidental.

Wrong answers often:

• Attack something irrelevant (a detail not needed for the causal claim).
• Provide a fact that doesn’t change the likelihood of the causal link.

Quantifier trap (some/most/all)

Be careful about scope. If a conclusion says “all,” one counterexample is devastating. If it says “most,” one counterexample usually isn’t enough unless it suggests a pattern.

Necessary vs. sufficient confusion

Some arguments treat a factor as sufficient when it might only be necessary (or vice versa). Weakening can exploit this by clarifying that the condition doesn’t guarantee the outcome.

Weaken in action (Example 1: Alternative cause)

Stimulus: “Since our company switched to a four-day workweek, productivity has increased. Therefore, the four-day workweek caused the productivity increase.”

Gap: Other changes might have increased productivity (new software, staffing changes, seasonal demand).

Answer choice (weakener): “At the same time the four-day workweek began, the company introduced a new project-management system designed to reduce wasted time.”

Why it weakens: It supplies a plausible alternative cause, reducing confidence that the workweek change is what did it.

Tempting wrong choice: “Many employees say they enjoy having a three-day weekend.”

• Enjoyment doesn’t show productivity wasn’t caused by the policy.

Weaken in action (Example 2: Representative sample)

Stimulus: “A website polled its visitors and found that 70% support Policy X. Thus, 70% of the public supports Policy X.”

Gap: Website visitors may differ from the public.

Answer choice (weakener): “The website is dedicated to advocating for Policy X and attracts visitors who are already interested in supporting it.”

Why it weakens: It suggests strong selection bias—making the poll a poor basis for a public-opinion conclusion.

Exam Focus

• Typical question patterns:
  • “Which of the following, if true, most seriously weakens the argument?”
  • “Which of the following, if true, casts the most doubt on the conclusion?”
  • “Which of the following, if true, undermines the reasoning above?”
• Common mistakes:
  • Treating “weaken” as “disprove” and overlooking answers that significantly reduce likelihood without killing the conclusion.
  • Choosing an answer that contradicts a premise rather than attacking the link to the conclusion (sometimes that works, but often it’s a trap that doesn’t touch the reasoning).
  • Missing quantifier scope—thinking one example defeats a “most” claim, or failing to notice that one counterexample does defeat an “all” claim.

Evaluate the Argument

What “evaluate” means on the LSAT

An Evaluate the Argument question asks you to choose information that would help you judge whether the argument’s conclusion is well supported. These questions are distinct because the correct answer is typically a yes/no question (or a fact that functions like a yes/no test) whose answer would swing your confidence depending on which way it comes out.

So instead of “Which fact strengthens?” you’re asked, in effect: “What would you most want to know to decide whether the reasoning works?”

Why evaluation questions are unique (and powerful)

Evaluate questions test a deeper critical-thinking skill: not just finding a flaw, but identifying the key uncertainty that the argument turns on.

In real life, this is what good decision-makers do. If someone claims, “This policy will reduce crime,” a good evaluator asks, “Compared to what? Controlling for what? How will we measure it?” You’re locating the pivotal missing information.

How evaluation works: find the hinge

Most arguments depend on one or two assumptions. An evaluate question targets the assumption, but instead of shoring it up (strengthen) or attacking it (weaken), you ask: What question would determine whether that assumption is true?

A practical way to frame it:

• Strengthen: “Give me a reason to believe the assumption.”
• Weaken: “Give me a reason to doubt the assumption.”
• Evaluate: “What information would tell me whether the assumption is true or false?”

The “two-way test” (the most reliable method)

A hallmark of the correct evaluate answer is that it matters in both directions:

• If the answer to the evaluate question is “yes,” the argument gets stronger.
• If the answer is “no,” the argument gets weaker.

When you test a candidate answer choice, ask yourself:

1. If this came out one way, would it help the argument?
2. If it came out the opposite way, would it hurt the argument?

If it only helps (or only hurts) no matter what, it’s usually not an evaluate answer—it’s more like a strengthen or weaken fact.

What evaluate answer choices usually look like

Because of that two-way nature, evaluate answers often:

• Ask whether a potential alternative cause exists.
• Ask whether a key term is being applied consistently.
• Ask whether a comparison group is appropriate.
• Ask whether the proposed solution has a necessary precondition.

They frequently use phrasing like:

• “Which of the following would it be most useful to know…?”
• “The answer to which of the following questions would be most helpful in evaluating…?”

Evaluate in action (Example 1: Causation hinge)

Stimulus: “People who drink green tea have lower rates of heart disease than people who do not. Therefore, drinking green tea reduces the risk of heart disease.”

What’s uncertain: Is green tea the cause, or do green-tea drinkers differ in other health-relevant ways?

Evaluate question (correct type): “Do green-tea drinkers and non-drinkers differ significantly in other habits that affect heart-disease risk, such as exercise and diet?”

Two-way test:

• If they do not differ, the causal claim looks more credible.
• If they do differ, the causal claim is much weaker (third-variable explanation).

Why other choices would be wrong: A choice like “Is green tea popular in some countries?” doesn’t directly bear on whether green tea causes lower risk.

Evaluate in action (Example 2: Plan/proposal hinge)

Stimulus: “To reduce missed appointments, the clinic should send text reminders to patients. Text reminders will reduce missed appointments because patients often forget their appointment times.”

What’s uncertain: Will reminders actually reach patients in time and be noticed? Forgetting is offered as the key cause—so the plan depends on that being true and on texts being an effective remedy.

Evaluate question (correct type): “Do most of the clinic’s patients have reliable access to text messaging and typically read texts shortly after receiving them?”

Two-way test:

• If yes, the plan is more likely to work.
• If no, the plan may fail even if forgetting is common.

A tempting wrong answer might be: “Are text reminders cheaper than phone calls?” That’s relevant to cost, but the argument’s stated support is about effectiveness at reducing missed appointments.

Common pitfalls in Evaluate questions

1. Confusing “evaluate” with “strengthen”

   • A pure strengthener might only push in one direction. Evaluate answers should be “hinge” information.

2. Picking something interesting but non-decisive

   • Evaluate information must connect to the argument’s core support. Interesting background facts usually won’t move the needle.

3. Missing what the argument actually claims

   • If the conclusion is modest, don’t evaluate a stronger claim. If the conclusion is about “reducing risk,” don’t evaluate “eliminating risk.”

Exam Focus

• Typical question patterns:
  • “The answer to which of the following questions would be most helpful in evaluating the argument?”
  • “Which of the following would it be most useful to determine in order to assess the reasoning?”
  • “Which question is most relevant to deciding whether the conclusion is properly drawn?”
• Common mistakes:
  • Choosing an answer that would only strengthen (or only weaken) rather than information that matters either way.
  • Evaluating a side issue (cost, popularity, moral desirability) when the argument’s reasoning is about effectiveness, causation, or representativeness.
  • Forgetting to apply the two-way test—if the opposite outcome wouldn’t matter, it’s usually not the correct evaluate choice.