How to solve GMAT Critical Reasoning Inference question

You’re having lunch with your friend Jane, and you suggest getting hot fudge sundaes for dessert; Jane tells you that she doesn’t eat hot fudge sundaes.  In real life, you could draw several valid inferences from this: she’s lactose intolerant, she has sensitive teeth and so can’t eat frozen desserts, she’s on a diet and trying to avoid sweets, or maybe she just doesn’t like ice cream or hot fudge. 

In real life, those would all be acceptable inferences, because the real-world definition of infer is to do any of the following:

1. to derive by reasoning; conclude or judge from premises or evidence: e.g., They inferred his anger from his heated denial.
2. (of facts, circumstances, statements, etc.) to indicate or involve as a conclusion; lead to.
3. to guess; speculate; surmise.
4. to hint; imply; suggest.

“Infer” is, as you can see, a word with fairly flexible meaning.  We most often use it in day-to-day life to mean “make an educated guess.”  If your friend Jane says she doesn’t eat hot fudge sundaes, you apply your existing knowledge about the possible reasons someone could have for not enjoying the hot fudge and ice cream deliciousness, and you make an educated guess as to what her reasons could be.  On the GMAT, however, “inference” has a different meaning.  Think of inferring as the process of deriving the strict logical consequences of assumed premises.

On the GMAT, therefore, if you are told that Jane doesn’t eat hot fudge sundaes, you can derive two logical consequences from that premise:

1. If Jane is eating, has eaten, or will eat something, it isn’t a hot fudge sundae, and
2. If someone is eating, has eaten, or will eat a hot fudge sundae, that person is not Jane.

The correct answer to an inference question on the GMAT will follow directly from the evidence provided; it is NOT merely an educated guess, but is instead the logical consequence of the assumed premises.

Notice that just based on six words—“Jane doesn’t eat hot fudge sundaes”—we can draw two possible inferences.  Now think of how many words you might see in the average GMAT question, and you’ll understand that inference questions, unlike other types of questions, don’t lend themselves well to prediction.  Trying to guess the correct inference being drawn from several sentences worth of statements is generally a waste of time.  Your best bet in approaching GMAT questions that ask for inferences is to use process of elimination, just as you would in sentence correction.  Eliminate answers that are just “educated guesses,” answers that aren’t necessarily true, answers that are too extreme, and of course, anything irrelevant.

Your answer will be the one choice that follows strictly from the statements in the question.

Let’s look at a sample GMAT-type question:

Q) XYZ Corporation has two divisions, both of which performed consistently over the last five years.  The Interment Services Division accounted for approximately 30% of the corporation’s transactions and 50% of the corporation’s profits; the Toxic Household Products Division accounts for the balance.

The statements above support which of the following inferences about XYZ Corporation over the last five years?

A. Measured in dollars, the total profits for XYZ Corporation have remained stable over the last five years.
B. Interment Services is an increasingly competitive field, while Toxic Household Products are a largely untapped market.
C. The Toxic Household Products Division yields a lower average profit per transaction than does the Interment Services Division.
D. XYZ Corporation’s Toxic Household Products line has remained consistent over the past five years.
E. Most families will, over a given five-year period, spend more money on Interment Services than on Toxic Household Products.

Only one of these answer choices MUST be true; let’s take a look at the options:

A. We only know about percentages, or proportions, so we can’t draw inferences about dollar amounts.
B. No information is provided about competition for either Interment Services or Toxic Household Products.  
C. This is the correct choice; Interment Services has a profit to transactions ratio of 50%:30%, or 5:3, while Toxic Household Products has a ratio of 50%:70%, or 5:7.  Therefore, the Toxic Household Products Division is doing more than twice as many transactions as the Interment Services Division, but yielding the same profits.
D. Product lines are not discussed, and therefore can’t be the subject of an inference.
E. Per-family spending is never mentioned, so we can’t infer anything about it.

There’s a pattern here: if it’s not mentioned, an inference can’t be drawn about it.  Inferences MUST be supported by the evidence provided; remembering this one concept will give you a solid start in conquering inference questions on the GMAT.