LSAT Logical Reasoning: How to Change an Argument’s Strength (and Measure It)

Strengthen the Argument

What “strengthening” means (and what it does not mean)

A Strengthen question asks you to choose the answer choice that—if true—makes the argument’s conclusion more likely to be true. The key is that LSAT strengthening almost never means “prove the conclusion with certainty.” Instead, you’re usually adding a missing piece that supports a shaky link, or you’re removing a reason to doubt the conclusion.

This matters because most Logical Reasoning arguments are not airtight deductions. They’re closer to real-world persuasion: someone gives a few reasons and then leaps to a broader claim. Strengthen questions test whether you can (1) see that leap and (2) know what kind of information would make it more reasonable.

A common misconception is that strengthening is about finding an answer choice that “sounds supportive” or repeats the conclusion. But repetition is not support. Support is information that changes the probability landscape—closing a gap, confirming an assumption, or ruling out a competing explanation.

The core mechanism: find the gap

Most strengthen questions are easiest when you treat the argument as a bridge:

  • Premises are one side of the river.
  • Conclusion is the other side.
  • The author’s assumption is the missing plank.

Assumption (in this context) means an unstated idea that must be true (or at least is taken for granted) for the premises to make the conclusion believable.

When you strengthen, you typically do one of these:

  1. Support an assumption (add the missing plank).
  2. Eliminate an alternative explanation (remove a reason the conclusion might be false even if premises are true).
  3. Provide a helpful additional premise (new evidence that points in the same direction).
  4. Clarify a definition or comparison (fix a mismatch in terms).

A reliable method is:

  1. Identify the conclusion (what the author is trying to establish).
  2. Identify the premises (what the author offers as support).
  3. Describe the gap in plain language: “This would make sense if…”
  4. Predict what would help—then match to choices.
Common strengthening “families” (especially on the LSAT)
Strengthening causal arguments

Causal arguments often look like: “A happened, then B happened, so A caused B.” The gap is that correlation or sequence does not guarantee causation.

To strengthen a causal conclusion, you might:

  • Rule out alternative causes (something else that could explain B).
  • Show the cause preceded the effect (timing support).
  • Show a mechanism (how A could produce B).
  • Add evidence from variation: when A increases, B increases; when A is removed, B decreases (controlled comparison).
Strengthening generalizations and predictions

These argue from a sample or past pattern to a broader claim.

To strengthen, you might:

  • Show the sample is representative.
  • Increase sample size or show the result is replicated.
  • Eliminate selection bias (the data weren’t cherry-picked).
  • Confirm that conditions will remain similar (for predictions).
Strengthening analogies

Analogies say: “X is like Y in relevant ways, so what’s true of X is true of Y.” Strengtheners:

  • Show more relevant similarities.
  • Show there are no relevant dissimilarities.
“Most strengthens” is comparative, not absolute

The question stem typically implies: pick the choice that helps more than the others. Sometimes every option is somewhat related, but only one targets the gap.

That’s why a powerful habit is prephrasing: before you look at answer choices, state what would help. You don’t need the exact wording—just the job the correct answer must do.

Strengthen examples (worked)
Example 1: Causal strengthening

Argument:
City officials note that after installing brighter streetlights in several neighborhoods, nighttime thefts decreased. Therefore, installing brighter streetlights causes a reduction in nighttime thefts.

Step 1: Conclusion
Brighter streetlights cause a reduction in nighttime thefts.

Step 2: Premise
After installation, thefts decreased.

Step 3: Gap
The decrease could be due to something else (more police patrols, economic changes, seasonal patterns). Also, maybe theft moved elsewhere.

What would strengthen?
Information ruling out key alternatives or showing theft decreases specifically where lights were installed while comparable areas did not change.

Strong strengthening statement (illustrative):
In otherwise similar neighborhoods that did not receive brighter streetlights during the same period, nighttime thefts did not decrease.

Why it works: it creates a comparison group, making the causal story more plausible.

Example 2: Strengthening a plan/policy recommendation

Argument:
A company should switch to Supplier Z because Supplier Z charges less per unit than the current supplier. So switching will reduce the company’s overall costs.

Gap
“Less per unit” doesn’t guarantee lower overall costs—there may be shipping fees, defect rates, switching costs, delays, or contract penalties.

What would strengthen?
A statement that addresses total cost, reliability, or hidden costs.

Strong strengthening statement (illustrative):
After including shipping costs and expected defect-related returns, Supplier Z’s total cost per usable unit is still lower than the current supplier’s.

Why it works: it directly patches a likely objection.

Exam Focus
  • Typical question patterns
    • “Which of the following, if true, most strengthens the argument?”
    • “Which option most supports the conclusion drawn above?”
    • “Which statement, if true, provides the most additional support?”
  • Common mistakes
    • Treating “strengthen” as “prove” and rejecting good answers because they don’t guarantee the conclusion.
    • Picking an answer that repeats or rephrases the conclusion rather than supporting it.
    • Falling for an answer that strengthens a premise generally but doesn’t strengthen the premise-to-conclusion link (the actual gap).

Weaken the Argument

What “weakening” means

A Weaken question asks you to choose the answer choice that—if true—makes the conclusion less likely. Again, you usually don’t need to destroy the argument completely. You just need to introduce information that undermines the reasoning: it creates doubt, exposes a bad assumption, or provides a plausible alternative.

This skill matters because weakening is the flip side of strengthening: if you understand what an argument needs to succeed, you also understand how it can fail. On the LSAT, weaken questions heavily reward noticing hidden assumptions and typical reasoning errors.

A frequent misconception is that weakening requires contradicting a premise directly. That’s one way, but not the only way—and often not the best way. Many correct weaken answers leave all premises intact but show that the premises don’t justify the conclusion.

The main weakening strategies

Think of weakening as pulling out support beams. Common approaches:

  1. Attack a premise: show a stated fact is false, unreliable, or misinterpreted.
  2. Attack the inference: show the conclusion doesn’t follow even if premises are true.
  3. Provide an alternative explanation: especially for causal arguments.
  4. Show a counterexample: the conclusion claims something general, but you show a case where it fails.
  5. Expose an equivocation: key term shifts meaning between premise and conclusion.
  6. Show the effect is reversed: maybe B causes A rather than A causing B.

A useful mnemonic for causal weakening is A.W.A.R.E.:

  • Alternative cause
  • Wrong direction (reverse causation)
  • Accident/Chance (insufficient evidence)
  • Randomization/selection issues (groups not comparable)
  • External factor (confounder)

(You don’t need to memorize labels, but the categories help you generate predictions.)

Weaken is still “if true”—so accept the new fact

On these questions, you must treat the correct answer as true even if it sounds surprising. Your job is not to argue with the answer choice; it’s to evaluate its impact.

Also, the correct weaken answer is often surgical: it targets the precise assumption. Wrong answers often sound “critical” but aim at something irrelevant.

Weaken examples (worked)
Example 1: Weakening a causal claim (alternative explanation)

Argument:
After the city added more bike lanes, traffic congestion decreased. Therefore, the new bike lanes caused the decrease in congestion.

Gap
Congestion could have decreased due to other changes (remote work, transit improvements, construction ending).

Weaken (illustrative):
During the same period, a large employer in the city permanently shifted most employees to remote work, reducing commuter traffic.

Why it works: it offers a plausible alternative cause for the effect, making the bike-lane conclusion less supported.

Example 2: Weakening a recommendation (hidden costs)

Argument:
We should replace in-person customer support with an automated chat system because automation will reduce staffing costs.

Conclusion
We should replace in-person support.

Gap
Cost savings might be offset by worse service, customer churn, legal risks, or increased downstream costs.

Weaken (illustrative):
Companies in the same industry that replaced most in-person support with chat automation saw a significant increase in customer cancellations.

Why it works: it attacks the implicit assumption that reducing staffing cost leads to better overall outcomes.

Example 3: Weakening a generalization (counterexample / representativeness)

Argument:
In a survey of 50 restaurant patrons, most said they prefer spicy dishes. So most people in this city prefer spicy dishes.

Gap
The sample may not represent the city.

Weaken (illustrative):
The survey was conducted at a restaurant known for spicy cuisine and located next to a university with many international students.

Why it works: it suggests strong selection bias—those patrons are not representative.

Subtle weakening: “even if premises are true, the conclusion doesn’t follow”

Some weaken answers don’t dispute any facts. Instead, they show that the premises are compatible with the negation (or near-negation) of the conclusion.

For example, if an argument concludes “Policy P will reduce costs,” a good weaken answer might show “Policy P reduces one category of cost but increases another by more.” No premise is directly contradicted, but the inference is damaged.

Exam Focus
  • Typical question patterns
    • “Which of the following, if true, most seriously weakens the argument?”
    • “Which option, if true, most undermines the conclusion?”
    • “Which statement, if true, casts the most doubt on the reasoning?”
  • Common mistakes
    • Choosing an answer that attacks a side issue rather than the argument’s core support.
    • Treating a merely “different” fact as weakening—even if it doesn’t make the conclusion less likely.
    • Over-focusing on contradicting a premise and missing more powerful options like alternative explanations or showing the plan has hidden costs.

Evaluate the Argument

What “evaluate” means—and how it differs from strengthen/weaken

An Evaluate the Argument question asks for information that would help you judge whether the argument’s reasoning is good. Unlike strengthen/weaken, you’re not asked to push the argument in one direction. You’re asked to identify what would matter most in assessing the argument.

On the LSAT, this almost always means:

  • The correct answer identifies a key assumption.
  • The answer choice is typically phrased as a question (or a statement that amounts to a question).
  • Depending on how that question is answered (yes vs no; high vs low), the argument becomes much stronger or much weaker.

So, Evaluate is like installing a “diagnostic test.” Strengthen is giving the argument medicine; weaken is giving it poison; evaluate is ordering the lab work that tells you whether it’s healthy in the first place.

The “two-way” test: the hallmark of correct Evaluate answers

A powerful way to recognize the right evaluate choice is to check whether it has two-way leverage:

  • If answered one way, it helps the argument.
  • If answered the other way, it hurts the argument.

Wrong answers often fail this because they only strengthen, only weaken, or are irrelevant no matter how answered.

How to solve Evaluate questions step by step
  1. Find the conclusion and premises as usual.
  2. Identify the assumption (what must be true for the reasoning to work).
  3. Ask: “What missing fact, if I knew it, would settle whether this assumption holds?”
  4. Choose the answer that most directly asks for that fact.

A practical trick: for each answer choice, imagine two possible responses—“Yes” and “No” (or “High” and “Low”). If both responses would have little effect, it’s not evaluative.

Common assumption types that Evaluate questions target
Causal assumptions

If the argument is causal, evaluate questions often ask about:

  • Whether there is an alternative cause.
  • Whether the cause and effect are correlated in the right way.
  • Whether the comparison group is similar.
Sampling/representativeness assumptions

If the argument generalizes from data, evaluate questions ask whether:

  • The sample is representative.
  • The measurement is reliable.
  • The trend is stable over time.
Plan/policy assumptions

If the argument recommends an action, evaluate questions ask whether:

  • The proposed plan has side effects.
  • The plan is feasible (cost, capacity, compliance).
  • The plan achieves the goal better than alternatives.
Evaluate examples (worked)
Example 1: Evaluating a causal argument

Argument:
Students who attend the school’s after-hours tutoring program have higher average grades than students who do not. Therefore, the tutoring program improves students’ grades.

Gap/Assumption
The tutoring program, rather than pre-existing differences, explains the higher grades. Maybe motivated students are the ones who attend.

Good Evaluate question (illustrative):
Were the students who chose to attend tutoring already earning higher grades than non-attendees before the program began?

Two-way leverage

  • If yes (they were already higher), the causal claim weakens.
  • If no (they were similar before), the causal claim strengthens.
Example 2: Evaluating a policy recommendation

Argument:
Our town should ban single-use plastic bags because doing so will significantly reduce litter in local parks.

Assumption
Plastic bags are a major contributor to park litter; the ban will reduce total litter rather than shift it.

Good Evaluate question (illustrative):
What proportion of the litter currently collected in local parks consists of single-use plastic bags?

Two-way leverage

  • If the proportion is high, the argument strengthens.
  • If the proportion is low, the argument weakens.
Example 3: Evaluating a generalization from a study

Argument:
In a study, people who drank two cups of coffee daily reported fewer headaches than those who drank none. So drinking coffee prevents headaches.

Assumption
Coffee intake is the relevant difference; reporting is reliable; other variables aren’t driving the effect.

Good Evaluate question (illustrative):
Did the study control for differences in sleep duration between coffee drinkers and non-drinkers?

Why it evaluates: if sleep differs systematically, that could explain headaches regardless of coffee.

How Evaluate connects to Strengthen/Weaken

If you get good at assumptions, these three question types start to feel like variations on one skill:

  • Strengthen: add a fact that supports the assumption or removes a competing explanation.
  • Weaken: add a fact that undermines the assumption or introduces a competing explanation.
  • Evaluate: ask for the fact that would tell you whether the assumption is true.

In practice, students often improve fastest by studying Evaluate questions because they force you to articulate the assumption cleanly—then Strengthen/Weaken becomes more predictable.

Exam Focus
  • Typical question patterns
    • “Which of the following would be most useful to know in evaluating the argument?”
    • “The answer to which question would be most helpful in assessing the reasoning?”
    • “Which consideration is most relevant to determining whether the argument is sound?”
  • Common mistakes
    • Picking a question that would be interesting but doesn’t affect the conclusion either way (fails the two-way leverage test).
    • Confusing Evaluate with Strengthen—choosing an option that, if answered one way, strengthens, but if answered the other way, is merely irrelevant rather than weakening.
    • Ignoring the conclusion and evaluating the topic generally instead of evaluating the specific inference the author makes.