GMAT Review: Critical Reasoning (Assumption, Strengthen/Weaken, Inference, Flaw, Boldface)

GMAT – Critical Reasoning (Assumption, Strengthen/Weaken, Inference, Flaw, Boldface)

GMAT Critical Reasoning – Assumption, Strengthen/Weaken, Inference, Flaw, Boldface

What This Is (1 short paragraph)

Critical Reasoning (CR) questions test your ability to evaluate arguments, identify hidden premises, and judge how new information affects conclusions. Each stem presents a short argument; the answer‑choice list asks you to pick an assumption, a strengthening/weakening factor, an inference, a flaw, or the bold‑face conclusion. Mastery is essential because CR accounts for roughly 30 % of the Verbal section and the logic patterns recur on every test.

Typical example:

Argument: “Because the city’s bike‑share program reduced traffic congestion by 12 % last year, expanding the program will solve the downtown parking crisis.”
Question: Which of the following, if true, most strengthens the argument?

Key Terms & Rules (8–12 bullets)

• Assumption – An unstated premise that must be true for the argument’s conclusion to hold.
• Strengthening – A statement that, when added, makes the conclusion more likely true; it must directly support the missing premise.
• Weakening – A statement that, when added, makes the conclusion less likely true; it usually attacks the hidden premise.
• Inference – A conclusion that logically follows from the given information; it never adds new facts.
• Flaw – A logical error in the argument (e.g., post hoc, hasty generalization, false cause).
• Bold‑face – The part of the argument that is explicitly or implicitly emphasized; it is the conclusion the question asks you to evaluate.
• Necessary vs. Sufficient – An assumption is necessary (must be true); a strengthening statement is sufficient (makes the conclusion true).
• “All‑but‑One” Rule – For Strengthen/Weaken questions, exactly one answer choice will do the job; the others will either be irrelevant or do the opposite.
• “No‑More‑Than‑One” Rule – For Inference questions, only one answer can be logically deduced; the rest over‑reach the given data.
• “Negate the Assumption” Test – To confirm an assumption, try to make it false; if the argument collapses, you have the right premise.
• “If‑Then” Formulation – Re‑write the argument as “If A, then B” to spot the hidden premise (A → B).

Step‑by‑Step / Process Flow (3–6 steps)

1. Read the stem twice – Identify the bold‑face (conclusion) and underline any explicit premises.
2. Paraphrase the argument in a simple “If A, then B” format; note any causal or comparative language.
3. Determine the question type (Assumption, Strengthen, Weaken, Inference, Flaw).
4. Apply the appropriate test:
5. Assumption: Negate each answer; the one that destroys the argument is the hidden premise.
6. Strengthen/Weaken: Plug each answer into the “If A, then B” skeleton; see which makes the conclusion more/less certain.
7. Inference: Eliminate any choice that adds new information; the remaining choice must be fully supported.
8. Flaw: Match the argument’s error to a known flaw pattern (e.g., “correlation ≠ causation”).
9. Cross‑check with the original text – Ensure the selected answer does not rely on outside knowledge or unstated facts.

Common Mistakes (3–5)

• Mistake: Treating a strengthening answer as an assumption because it “sounds plausible.”
  Correction: An assumption must be necessary; a strengthening statement is sufficient. Verify by negating the answer—if the argument still works, it’s not the assumption.

• Mistake: Selecting an inference that “seems likely” but actually adds a new fact.
  Correction: An inference can only rest on information explicitly or implicitly given. Remove any answer that introduces a detail not in the passage.

• Mistake: Overlooking the bold‑face cue and assuming the conclusion is the last sentence.
  Correction: The bold‑face may be a phrase inside a sentence; underline it in the stem and treat that exact phrase as the conclusion.

• Mistake: Confusing a flaw with a weakening factor.
  Correction: A flaw is a logical error in the argument’s structure; a weakening factor is external information that makes the conclusion less likely.

• Mistake: Assuming “because” always signals a causal link.
  Correction: GMAT writers sometimes use “because” loosely; test the causal claim with a flaw analysis (e.g., post‑hoc).

Exam Insights (2–4)

1. Most‑tested hidden premise: “X causes Y” without any other mediating factor. Spot it by asking, “What must be true for X to lead to Y?”
2. Bold‑face trap: The conclusion is often embedded (“…which means the city will…”) rather than a stand‑alone sentence.
3. Strengthen vs. Weaken symmetry: The correct answer will be the only choice that, when added, changes the probability of the conclusion in the direction asked.
4. Flaw patterns appear in ~15 % of CR items; the most common are post hoc (assuming temporal order = causation) and insufficient sample (drawing a universal claim from a tiny data set).

Quick Check Questions (2–3)

1. Assumption –
   Argument: “Because the new software reduced processing time by 20 % in the pilot plant, the company should roll it out to all factories.”
   Which of the following is an assumption?
   
   A) The pilot plant’s workload is typical of all factories.
   
   B) The software costs less than the current system.
   
   C) Employees will accept the new software.
   
   D) The pilot plant’s data were collected over a full year.
   Answer: A – If the pilot plant is not typical, the conclusion fails.

2. Strengthen –
   Argument: “If the city bans single‑use plastic bags, litter on streets will drop dramatically.”
   Which statement, if true, most strengthens the argument?
   
   A) 80 % of street litter is currently plastic bags.
   
   B) Residents will switch to reusable bags.
   
   C) Other cities that banned plastic bags saw a 30 % litter reduction.
   
   D) Plastic bags decompose slower than paper.
   Answer: C – Provides external evidence that the ban leads to a measurable drop.

3. Flaw –
   Argument: “Since sales rose after the company hired a new CEO, the CEO must be responsible for the increase.”
   What is the flaw?
   
   A) Hasty generalization
   
   B) Post hoc ergo propter hoc
   
   C) False dilemma
   
   D) Circular reasoning
   Answer: B – Assumes temporal succession equals causation.

Last‑Minute Cram Sheet (10 one‑liners)

1. ⚠️ Assumption ≠ Strengthening – Assumption = necessary; Strengthening = sufficient.
2. Bold‑face is always the conclusion, even if it’s a phrase inside a sentence.
3. Negate the assumption to test it; if the argument still holds, you’ve chosen the wrong premise.
4. Only one answer will make the conclusion certainly true (strengthen) or certainly false (weaken).
5. Inference must be deducible from the text; never add outside facts.
6. Flaw questions require you to name the type of logical error, not the missing premise.
7. “Because” ≠ causation – always check for a post‑hoc flaw.
8. All‑but‑One Rule – for Strengthen/Weaken, eliminate any answer that is merely “consistent” but not decisive.
9. If‑Then re‑write: “If A, then B” → hidden premise = “A is sufficient for B.”
10. Time‑Saver: Scan the answer list first; cross out any choice that contains a word not in the stem (e.g., “always,” “never”).

Good luck—treat each CR item as a mini‑logic puzzle, and you’ll turn those traps into stepping stones!