LSAT Logical Reasoning: Inference vs. Assumption

You’re having lunch with your friend Bob, and you suggest splitting an order of onion rings. Bob says that he doesn’t eat onion rings. In real life, you could draw several valid inferences from this: maybe Bob doesn’t like onions, maybe he’s watching his weight so he’s avoiding fried foods, maybe he doesn’t like crunchy appetizers. 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 Bob says he doesn’t eat onion rings, you apply your existing knowledge about the possible reasons someone could have for not enjoying a tasty breaded and deep-fried ring of onion, and you make an educated guess as to what his reasons could be. On the LSAT, however, “inference” has a different meaning. Think of inferring as the process of deriving the strict logical consequences of assumed premises.
On the LSAT, therefore, if you are told that Bob doesn’t eat onion rings, you can derive two logical consequences from that premise:

1. If Bob is eating, has eaten, or will eat something, it isn’t an onion ring, and
2. If someone is eating, has eaten, or will eat an onion ring, that person is not Bob.

The correct answer to an inference question on the LSAT 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 five words—“Bob doesn’t eat onion rings”—we can draw two possible inferences. Now think of how many words you might see in the average LSAT 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 LSAT 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.

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

1. We only know about percentages, or proportions, so we can’t draw inferences about dollar amounts.
2. No information is provided about competition for either Cremation Services or Poisonous Cleaning Supplies.
3. This is the correct choice; Cremation Services has a profit to transactions ratio of 50%:30%, or 5:3, while Poisonous Cleaning Supplies has a ratio of 50%:70%, or 5:7. Therefore, the Poisonous Cleaning Supplies Division is doing more than twice as many transactions as the Cremation Services Division, but yielding the same profits.
4. Product lines are not discussed, and therefore can’t be the subject of an inference.
5. 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 LSAT.

LSAT logical reasoning questions often ask you to identify the assumption of an argument. The first step in doing that successfully is understanding what, exactly, they mean by “assumption.” An assumption in LSAT-speak is the unstated link somewhere in the chain of evidence and conclusion. Finding the assumption means, basically, finding that gap in the argument and filling it.
Assumptions can be roughly divided into “necessary” and “sufficient,” and your approach to tackling an assumption question depends in part on which kind of assumption you’re dealing with. A necessary assumption MUST be true in order for the conclusion to follow logically based on the evidence presented.

Here, you’re looking at finding the unstated idea that MUST BE TRUE in order for the argument to work logically. Now, a few of these choices support the argument’s conclusion. But only one of them is actually necessary to the argument. Let’s looks at them one at a time.
a) Isaac gets good grades in all of his math classes.
This isn’t an assumption of this argument at all. Isaac’s other math classes are outside the scope of the argument, since they are addressed in neither the evidence nor the conclusion.
b) All boys named Isaac are smart.
This choice would certainly support the conclusion; if this were true, then the conclusion would HAVE to be true. But is this statement NECESSARY to the conclusion? No. Other boys named Isaac don’t have any necessary significance to this argument. So this is not a good choice.
c) Isaac wouldn’t wear glasses if he wasn’t smart.
Again, this choice would be SUFFICIENT to make the argument’s conclusion follow from the evidence.  But is it necessary? No. So we’ll bypass this one.
d) Some people who get A’s in algebra are smart.
This is the correct choice, because it MUST be true in order for the evidence to follow logically from the conclusion. What if this wasn’t true, and no one who got an A in algebra was smart? If that were the case, then the conclusion would not be true, based on the evidence that Isaac gets A’s in algebra.
e) Everyone who gets an A in algebra is smart.
Once more, this choice is sufficient to support the conclusion, but it’s not necessary.  So it’s not the correct answer to the question that is being posed.
Now, hopefully you noticed that the correct answer here is the least extreme relevant statement. That doesn’t always have to be the case, but for questions that ask for necessary assumptions, it’s a good general guideline. Be wary of answer choices that are extreme; they will often be sufficient, but not necessary, and will therefore trick test-takers who aren’t careful in evaluating what exactly the question has asked them to find.
But what if the question paired with that argument looked more like this?
Which of the following assumptions, if true, best supports the conclusion above?
Well, in that case, the answer choices would look more like these:
a) Isaac gets good grades in all of his math classes.
b) All boys named Isaac are smart.
c) Isaac gets A’s in his geometry class.
d) Some people who get A’s in algebra are smart.
e) Some people who don’t wear glasses are smart.
Just as in the last example, choice a) is not relevant to the argument as an assumption. But here, choice b) is the correct answer, because if that statement is true, then the conclusion is absolutely true. Choices c) and e) are irrelevant in the same way that choice a) is, since physics and people who don’t wear glasses aren’t at issue here. Now, choice d) is NECESSARY to the argument, but it is not the BEST support to the conclusion. Even if it IS true that some people who get A’s in algebra are smart, that doesn’t guarantee that Isaac is.

On Test Day

Keep a close eye on what the question is asking for, and read accordingly.

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