LSAT Logical Reasoning: How to Change an Argument’s Strength (Strengthen, Weaken, Evaluate)

Strengthen the Argument

What “strengthening” means on Logical Reasoning

On LSAT Logical Reasoning, an argument is a set of statements where some statements (the premises) are offered as support for another statement (the conclusion). A Strengthen question asks you to choose an answer choice that, if taken as true, makes the conclusion more likely to follow from the premises.

Two points that matter immediately:

1. Strengthening is almost never about making the conclusion certain. LSAT arguments are usually short and leave gaps. Your job is to make the reasoning better, not perfect.
2. The correct answer is judged relative to the other choices. Several answers may sound helpful, but one will provide the most support in a logically relevant way.

Why strengthening matters (and what it tests)

Strengthen questions test whether you can see the structure of reasoning—especially the hidden gap between premises and conclusion. In most real-life persuasion (ads, editorials, workplace proposals), people don’t lay out every necessary step. They rely on assumptions. The LSAT is measuring whether you can:

• Identify what the argument needs to be true
• Recognize what kind of evidence would shore up that need
• Ignore information that is interesting but irrelevant

Strengthen questions are also closely connected to other Logical Reasoning skills:

• Assumption questions: strengthening often looks like supplying an assumption (or support for it).
• Causal reasoning: strengthening often means ruling out alternative causes.
• Sampling/generalization: strengthening often means making the sample more representative.

How strengthening works (a practical step-by-step method)

A reliable approach is:

1. Find the conclusion. Look for indicator words (therefore, thus, so) or the “main point” the author is trying to prove.
2. List the premises. What reasons are offered?
3. Describe the gap. Ask: “Even if the premises are true, what could still make the conclusion false?” That “could” is the gap.
4. Predict the kind of help needed. You don’t need the exact answer—just the type (e.g., “rule out another cause,” “show this sample is typical,” “show these two things really correlate”).
5. Check answer choices for relevance first, then strength. Wrong answers often fail because they don’t affect the conclusion. Right answers make the conclusion more plausible.

A useful mental model is that every argument has a “bridge” from premise to conclusion. Strengthening answers either:

• Reinforce the bridge (directly support the inference)
• Repair a weak plank (support an assumption the bridge relies on)
• Block attacks (eliminate a key alternative explanation)

Common “families” of strengthening moves

1) Support a necessary assumption

Many LSAT arguments rely on something like: “If the premises are true, then the conclusion follows as long as ___ is also true.” Strengthening often supplies evidence for that blank.

Example pattern:

• Premise: This medicine reduced symptoms in a small study.
• Conclusion: It will reduce symptoms in the general population.
• Hidden need: The study subjects are representative; the study was well-controlled.

A strengthening answer might say the study was double-blind, or subjects matched the broader population.

2) Rule out alternative explanations (especially in causal arguments)

If an argument says $A$ caused $B$, a strong strengthening move is to show that:

• Other plausible causes were absent
• $A$ happened before $B$
• When $A$ changes, $B$ changes (with controls)

This doesn’t prove causation, but it makes the causal claim more credible.

3) Strengthen by adding a missing link (conditional/quantified reasoning)

Sometimes the premises give you: “All $X$ are $Y$” and “All $Y$ are $Z$,” and the conclusion says something about $X$ being $Z$. Strengthening might confirm a relationship that wasn’t actually established.

Be careful: LSAT strengthening is usually not formal-proof completion; it’s often about making an inference more defensible.

4) Confirm the reliability of evidence

If the premises rely on testimony, a survey, measurements, or data, strengthening can come from establishing that the method was sound:

• The survey used random sampling
• The measuring instrument was calibrated
• The witness had good conditions to observe

5) Clarify definitions and remove ambiguity

A surprising number of arguments fail because a key term shifts meaning. A strengthening answer might lock the meaning in place.

Example: An argument about “success” might rely on whether success means “profit” vs “customer satisfaction.” If the correct answer clarifies that the author’s meaning is consistent, it can strengthen.

Strengthen in action: worked examples

Example 1 (causal strengthen)

Stimulus:

After the city installed brighter streetlights, nighttime car thefts decreased. Therefore, the brighter streetlights caused the decrease in nighttime car thefts.

Step 1: Conclusion: Brighter streetlights caused the decrease.

Step 2: Premise: After installation, thefts decreased.

Step 3: Gap: Many other factors could explain the decrease (more policing, seasonal changes, economic changes, a new anti-theft technology trend). Also, “after” doesn’t necessarily mean “because.”

What would strengthen? Evidence that rules out a major alternative or shows thefts dropped specifically where lights changed.

Correct strengthening answer (example):

In similar nearby areas where streetlights were not changed, nighttime car thefts did not decrease during the same period.

Why this helps: it targets a core alternative explanation (a general trend) by providing a comparison group.

Example 2 (generalization strengthen)

Stimulus:

A survey of 200 customers who visited our website yesterday found that most thought the site was easy to navigate. So, most of our customers think the site is easy to navigate.

Gap: “Yesterday’s visitors” might not represent “all customers.” Maybe only tech-savvy users visited yesterday.

Correct strengthening answer (example):

The 200 customers surveyed were randomly selected from all customers who visited the site over the past month.

Why this helps: it makes the sample more representative of the population in the conclusion.

What usually goes wrong when students try to strengthen

A common mistake is to pick an answer that “sounds supportive” but doesn’t actually touch the reasoning. Another is confusing strengthening with proving—you don’t need a slam-dunk, you need the option that most increases likelihood.

Also watch for “strengtheners” that introduce a new topic. Even if the new information is positive, it can be irrelevant if it doesn’t connect premises to conclusion.

Exam Focus

• Typical question patterns:
  • “Which one 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 strongest support for the claim that…?”
• Common mistakes:
  • Choosing an answer that strengthens a premise (makes it sound nicer) but doesn’t strengthen the inference to the conclusion.
  • Overvaluing extreme language (“proves,” “guarantees”) instead of relevance.
  • Missing that the argument is causal or a generalization, leading you to strengthen the wrong “gap.”

Weaken the Argument

What “weakening” means on Logical Reasoning

A Weaken question asks you to choose an answer choice that, if taken as true, makes the argument’s conclusion less likely to follow from the premises. Like strengthening, weakening is usually about probability, not certainty. The right answer typically introduces information that exposes a flaw, highlights an alternative explanation, or undermines an assumption.

A key mindset shift helps: to weaken, you don’t need to show the conclusion is false—you need to show the argument’s support is insufficient.

Why weakening matters (and how it connects to other LR skills)

Weakening trains you to find vulnerability in reasoning. In real life, critical thinking often looks like this: you hear a claim with evidence, then ask, “But what if…?” The LSAT rewards your ability to identify the most damaging “what if.”

Weaken is closely tied to:

• Flaw questions: weakening often exploits the same common flaws.
• Assumption questions: weakening often negates or attacks a necessary assumption.
• Causal reasoning: weakening often gives an alternative cause or reverses causation.

How weakening works (a practical step-by-step method)

Use a parallel method to Strengthen, but aim your “gap” in the opposite direction:

1. Identify conclusion and premises.
2. Find the assumption(s). Ask: “What must be true for these premises to justify that conclusion?”
3. Attack the weakest point. You can weaken by:
   • Undermining a premise
   • Undermining the link from premises to conclusion
   • Introducing an alternative explanation
   • Providing a counterexample to a general claim
4. Prefer direct hits over side comments. The best weakeners usually target the argument’s core mechanism.

A useful shortcut: if you can articulate the argument’s main assumption, a strong weakener often looks like “Actually, not that.”

Common “families” of weakening moves

1) Provide an alternative explanation (especially for causal claims)

If the argument claims $A$ caused $B$, a classic weakener says: “$C$ caused $B$” or “$B$ caused $A$” (reverse causation).

Example:

• Claim: “Taking vitamin X caused improved memory.”
• Weakener: “Participants also started sleeping more during the study.”

2) Show the effect exists without the cause (or the cause without the effect)

Still in causation:

• If $B$ happens even when $A$ doesn’t, $A$ is less likely to be the cause.
• If $A$ happens but $B$ doesn’t, $A$ is less likely to be sufficient.

3) Attack representativeness (for survey/generalization arguments)

If the argument generalizes from a sample to a population, weaken by showing:

• The sample is biased
• The sample size is too small for the conclusion drawn
• The selection process excludes key groups

Often the best weakener points to a systematic bias, not just “maybe it’s wrong.”

4) Undermine a key premise

Sometimes the simplest weakener is: a core fact the argument relies on is doubtful.

Be careful: on LSAT weaken questions, you don’t get to say “the premise might be false.” The answer choice must give you new information that (if true) makes that premise less credible.

5) Exploit equivocation (a shift in meaning)

If the argument uses a term in one sense in the premises and another in the conclusion, a good weakener may highlight that the two senses are different.

6) Show that the proposed plan has a hidden cost or constraint

A common real-world argument type is a proposal: “Do $X$; it will solve problem $Y$.” Weakening might point out:

• $X$ creates another problem
• $X$ can’t be implemented as described
• $X$ doesn’t address the real cause of $Y$

Weaken in action: worked examples

Example 1 (causal weaken)

Stimulus:

After the company introduced free snacks, employee productivity increased. Therefore, providing free snacks increases employee productivity.

Gap: Many changes could have happened at the same time; productivity might have been going up anyway.

Strong weakener (example):

At the same time the snacks were introduced, the company also replaced its outdated computers with faster ones.

Why this works: it gives a plausible alternative cause tied directly to productivity.

A weaker wrong answer might be: “Employees like the snacks.” That supports the idea that snacks are appreciated, but doesn’t address whether snacks caused productivity.

Example 2 (generalization weaken)

Stimulus:

In a poll of readers of TechToday magazine, 80% said they support expanding public transportation funding. So, most residents of the city support expanding public transportation funding.

Gap: Readers of TechToday may not represent city residents.

Strong weakener (example):

TechToday readers are disproportionately young professionals who are far more likely than other residents to use public transportation.

Why this works: it identifies a specific, directional sampling bias that would inflate support in the poll.

What usually goes wrong when students try to weaken

A frequent mistake is choosing an answer that contradicts the conclusion in a broad way but doesn’t actually engage the reasoning.

For instance, if the conclusion is “Policy $P$ will reduce costs,” a tempting wrong answer is “Some people oppose policy $P$.” Opposition doesn’t mean it won’t reduce costs.

Another common error is picking a choice that weakens only under additional assumptions (“This would weaken it if we also assume…”). On LSAT, the correct answer should weaken as stated, given ordinary background assumptions.

Exam Focus

• Typical question patterns:
  • “Which one of the following, if true, most seriously weakens the argument?”
  • “Which of the following, if true, casts the most doubt on the conclusion?”
  • “Which answer choice most undermines the reasoning?”
• Common mistakes:
  • Confusing “disagrees with the conclusion” with “weakens the support for the conclusion.”
  • Overusing hypothetical doubt (“might,” “could”) instead of selecting an answer that creates a concrete logical problem.
  • Missing the argument type (especially causation), leading you to choose a weakener that doesn’t address alternative explanations.

Evaluate the Argument

What an “Evaluate” question is really asking

An Evaluate the Argument question asks you to pick the question (or piece of information) that would be most useful for deciding whether the argument is strong or weak.

This is different from Strengthen/Weaken in an important way:

• In Strengthen/Weaken, the correct answer pushes the argument in a direction.
• In Evaluate, the correct answer helps you judge—depending on how it’s answered, it could strengthen or weaken.

So an evaluate answer is like a diagnostic test. You’re looking for information that targets the argument’s most important uncertainty.

Why evaluate questions matter

Evaluate questions test mature critical reasoning: not just arguing for or against, but identifying what evidence would actually settle the dispute.

In real life, this is the difference between debating endlessly and asking the one question that matters:

• “Does this study control for other variables?”
• “Is this witness in a position to know?”
• “Is the sample representative?”

On the LSAT, the evaluate task is strongly connected to finding assumptions. The best evaluating question often points straight at a key assumption—because if that assumption is true, the argument looks better; if false, it looks worse.

How evaluate questions work (the “yes/no” test)

Most evaluate questions can be handled with a powerful technique:

1. Identify the conclusion and premises.
2. Identify the key gap or assumption.
3. Look for an answer choice that asks about that gap in a way that has two clear opposite outcomes.
4. Apply the yes/no test:
   • If the answer to the question is “Yes,” does it strengthen?
   • If the answer is “No,” does it weaken?

If both directions matter, you’ve likely found an evaluate answer.

Be careful: not every evaluate answer is literally yes/no in wording, but the logic should function that way—two plausible outcomes that would move your confidence in opposite directions.

Common types of evaluate prompts

1) Evaluating causal arguments

If the argument is causal, the evaluating question often asks about:

• Alternative causes
• Whether the cause preceded the effect
• Whether the effect occurs without the cause
• Whether there is a control group or comparison

A classic evaluate question template is:

• “Did anything else change at the same time?”

If yes, argument weakens; if no, argument strengthens.

2) Evaluating surveys and generalizations

Here, the evaluating question is usually:

• “Is the sample representative of the population?”
• “How were respondents selected?”
• “Was the question wording neutral?”

If representative/neutral, argument strengthens; if biased/leading, it weakens.

3) Evaluating plans and proposals

Plans often assume that a proposed action will be feasible and will address the real problem. Useful evaluating questions ask:

• “Can the plan actually be implemented with available resources?”
• “Does it address the main cause of the problem?”
• “Will it create offsetting harms or costs?”

Again, a “yes” tends to strengthen; a “no” tends to weaken.

Evaluate in action: worked examples

Example 1 (evaluate a causal claim)

Stimulus:

Students who attend the school’s free after-school tutoring program tend to have higher math grades than students who do not. Therefore, the tutoring program improves students’ math grades.

What’s the gap? The argument assumes tutoring causes higher grades, but it could be selection: motivated students are more likely to attend.

Best evaluating question (example):

Are students who choose to attend the tutoring program, on average, more motivated to improve their math performance than those who do not attend?

Yes/no test:

• If yes (attendees are more motivated), the causal claim is weaker (selection effect).
• If no (motivation is similar), the causal claim is stronger.

Notice what makes this a good evaluate question: it directly targets the biggest alternative explanation.

Example 2 (evaluate a policy proposal)

Stimulus:

The city should replace many downtown parking spaces with protected bike lanes. This will reduce traffic congestion.

Gap: The argument assumes bike lanes will shift enough commuters from cars to bikes (or otherwise improve flow) to reduce congestion.

Best evaluating question (example):

Would a substantial portion of current downtown car commuters be willing and able to switch to biking if protected bike lanes were added?

Yes/no test:

• If yes, the plan is more likely to reduce congestion.
• If no, congestion likely won’t improve (and might worsen if lanes reduce road capacity).

What usually goes wrong with evaluate questions

A very common mistake is choosing a question that is “generally relevant” but not decisive. Evaluate questions reward pinpointing.

For example, asking “Do people like tutoring?” is about attitudes, not whether tutoring causes grade improvement. Even if students like it, it may not work; even if they dislike it, it might still help.

Another mistake is choosing a question that only matters in one direction. If a “yes” would strengthen but a “no” wouldn’t really weaken (or vice versa), it’s often not the best evaluator.

How Strengthen/Weaken/Evaluate fit together

It helps to see these as three ways of interacting with the same underlying structure (premises → conclusion):

If you can consistently articulate the gap, you’re most of the way to mastering all three.

Exam Focus

• Typical question patterns:
  • “The answer to which one of the following questions would be most helpful in evaluating the argument?”
  • “Which of the following would it be most important to know in order to assess the argument?”
  • “Which question would be most relevant to determining whether the conclusion is valid?”
• Common mistakes:
  • Picking a question that is interesting but doesn’t create a meaningful strengthen-vs-weaken split.
  • Missing the argument’s main reasoning type (causal, survey, plan), which makes it harder to ask the right diagnostic question.
  • Treating Evaluate like Strengthen—choosing something that only supports the argument rather than something that would test it.