Logical Reasoning: A Brief Introduction to Question Types

Posted by Spencer on 10/19/18 5:26 PM

Question Types Logical Reasoning LSAT

The following are some of the question types you will see on the LSAT Logical Reasoning Section. Again, there are often questions that appear that are not standard in the exam; however, the following types are the most common questions asked. They are (roughly) listed in order of frequency. 

Necessary Assumptions

Recall that a necessary assumption is needed for the argument to make sense, but does not necessarily allow it to work. For example, if I said:

Every computer is prone to water damage. 

I would have to assume that all Microsoft computers are prone to such damage. Does this allow my argument to work? Absolutely not. It could be the case that Apple computers (or another brand) are waterproof and not prone to such damage. Thus a necessary assumption does not always allow an argument to be properly drawn.

Examples of the way these questions are worded include:

“The argument depends on which of the following assumptions”

“Which of the following, if true, is necessary for the argument to be properly drawn?”

“In order for the argument to be properly drawn, which of the following must be true?” 

Notice the last example. This is different than asking for an assumption that allows the argument to be drawn. An assumption that must be true is something that the argument needs in order to be properly drawn, not something it doesn’t need. For example, if I said:

All floors in this home are built with a sturdy wood.

an assumption that might allow the argument to be properly drawn is:

Pinewood is sturdy and every floor and ceiling in the home is built with pinewood.

While this allows my argument to be drawn, it is not a necessary assumption because it over-assumes. For example, what if chestnut is also a sturdy wood? I would not need to assume a floor was built with pinewood in order for it to be built with a sturdy wood. Also, why would I need to assume that the ceilings in the home are built with a sturdy wood? It could be the case that the floors are all built with sturdy wood, but the ceilings are not. While this assumption is sufficient to allow my argument to be drawn, it brings in too much information and thus is not a necessary assumption.

Right answers on necessary assumptions will directly address a flaw in the argument. After you read the argument, you should have at least some idea of an assumption it missed. For example if I said:

Every hole in the yard is deep because it was dug by a dog.

an assumption I would need to make is:

Every dog can dig a deep hole.

Again, this is an assumption I would want to think of before I go the answer choices. Always ask yourself exactly what the argument missed in its reasoning because that will likely be the right answer (or very close to it). Wrong answers on necessary assumptions generally over-assume the information needed, or do not address the argument. For example, if I said:                      

All mints in checkered boxes are too strong.

an example of an answer choice that does not address the argument is:

All mints sold in convenience stores are too strong.

Notice that I do not know whether mints in checkered boxes are sold in convenience stores. Thus assuming that those mints are too strong does not address my original argument, and thus does not need to be assumed.

Sufficient Assumptions (also called “Justify the Principle”) 

A sufficient assumption is an assumption that allows the argument to work. Examples of the way these questions are worded include:

“Which of the following, if true, allows the argument to be properly drawn?”

“Which of the following, if true, most justifies the action taken in the argument above?”

“Which of the following, if true, most justifies the principle given in the argument above?”

There are two types of questions that deal sufficient assumptions: questions that ask you to justify an action or principle and questions that ask you to make an argument work. The second type is very similar to necessary assumptions, with the exception that a sufficient assumption may over-assume or include unnecessary information. For example, if I said:

All professional soccer games last at least two hours.

a sufficient assumption may be:

Every soccer game lasts more than three hours.

Notice that my assumption over-assumes in two ways. First, I do not need to assume that every soccer game has lasted more than two hours, as I am only concerned with professional soccer games. Second, I only need to assume that professional soccer games last more than two hours, not three. However, this assumption is a sufficient assumption because it allows the argument to be drawn. Even though it includes additional information, it gives enough information to allow my argument to hold.

The first type of sufficient assumption questions, justifying a principle or action, deals strictly with normative arguments (i.e. what someone should or ought to do). For these questions, right answers must justify an action or principle based on the reasoning given in the stimulus. For example, if I said: 

Stealing is wrong because it harms other people. 

something I must assume is:

Harming other people is always wrong.

Notice that the reasoning of the argument states that something is wrong because it harms others. Thus if something harms others, it must be wrong. But is it always wrong to harm others? That is not clear in the argument, and must be addressed in any answer choice that attempts to justify it. Examples of wrong answer choices include assumptions that fail to address the scope of the argument, or fail to address the action taken in the stimulus. For example, if I said: 

George was wrong in violating the company’s policy on tardiness because he knew being late was against the company’s rules. 

an assumption that fails to address the scope of the argument may be:

Being tardy is always wrong.

This is a very tempting wrong answer because it supports that George’s action was wrong. However, it does not address the reasoning given in the argument. The argument is saying that an action is wrong if the person taking it knew it was against the rules. This answer choice does not address what George knew or did not know, merely that this particular action was wrong. An answer choice that justifies the argument would very clearly state that knowingly violating the company’s policy is wrong.

Another type of common error on justification questions is failing to justify an action taken in the stimulus. For example, take the following principle and action:

Principle:  The government should not restrict the free will of its citizens, unless it does so to protect its citizens’ interests.

Action: Government Z briefly took away its citizens’ right to vote, because doing so would protect the interests of its citizens.

Notice the word unless in the principle above. On the LSAT, unless simply means if not. So an easy way to word the principle above would be:

The government should not restrict the free will of its citizens if it does not do so to protect their interests. 

Remember that a conditional statement always begins with the word if. Notice how this might be diagrammed:

Action does not protect interests → should not restrict free will. 

and the contrapositive:

Should restrict free will → action does protect interests.

But notice that the action taken in the stimulus cannot be justified by either of these statements. The action in the stimulus implies that:

Action protects interests → should restrict free will.

but neither the original statement nor its contrapositive can support this. Thus a right answer choice must clearly state that if an action protects the interests of its citizens, it is therefore justified to restrict their free will.

Strengthen/Weaken the Argument

It is important to remember that the answer choices in strengthen and weaken questions always contain four wrong answers. Thus it is never helpful to look for the “best” answer, but instead eliminate the wrong answer choices and then select the correct answer choice. Examples of the way these questions are worded include:

“Which of the following, if true, lends most support to the argument above?”

“Which of the following, if true, most strengthens/weakens the argument above?”

“Each of the following, if true, strengthen/weaken the argument above EXCEPT:”

Notice the last example. While many strengthen/weaken questions ask you to look for one assumption that supports or weakens the argument, a few may ask you to find four assumptions that support/weaken and then select the final assumption as your answer. Note that this answer may not support or weaken, but may simply fail to address the argument. For example, if you were given the following question:

  1. Brenda paid her taxes on time this month because she was not contacted by the revenue service.

Each of the following, if true, supports the argument above EXCEPT:

  • The revenue service usually contacts people who fail to pay their taxes within a few weeks.
  • The revenue service recently upgraded to an automatic phone service that calls all delinquent tax payers automatically to remind them to pay their taxes.
  • Brenda usually mails a check at the first of every month to pay her utility bill and water bill.
  • Revenue service officers frequently mail out reminders to those who have not paid their taxes on time.
  • Brenda has not participated in a survey conducted by the revenue service this month, targeting delinquent tax payers in her district.

Notice that four of the answer choices lend some support to the argument. However, (C) neither weakens nor supports the argument. Thus the best strategy in approaching this question is to look for an answer choice that either weakens or does not address the argument.

Common wrong answer choices in weaken/strengthen questions are assumptions that use relative distinctions (i.e. comparing something to something else). For example, if I said:

All buildings in downtown Seattle are tall.

an assumption that fails to support this statement may be:

The buildings in downtown Seattle are taller than those in Memphis.

This is a tempting wrong answer because it does make a claim about the height of the buildings in Seattle. But what if all of the buildings in Memphis are short? Could we still say that the buildings in Seattle are tall? Absolutely not. Simply because something is taller, heavier, faster, etc. than something else, it does not mean that it itself is tall, heavy, or fast. Comparing two objects in order to make a claim about one of them only works if you know the properties of both. For example, if I know that all of the buildings in Memphis are tall, then I would know that the buildings in Seattle are tall as well. However, the assumption above does not state this, so it cannot be assumed.

Another common error on strengthen/weaken questions is misinterpreting the words most and some. On the LSAT, the word most means more than half, and the word some means at least one. For example, if I said:

Most gorillas have twenty teeth.

then I assume that more than 50 percent of gorillas have twenty teeth. However, if I said:

Most gorillas have twenty teeth, and most gorillas who have twenty teeth are in captivity.

does it follow that most gorillas are in captivity? Absolutely not. For example, if 51 percent of gorillas have twenty teeth, and most of those 51 percent are in captivity, it follows then that greater than 50 percent of that 51 percent who have twenty teeth are in captivity. That could mean that all 51 percent with twenty teeth are in captivity, but it doesn’t make a claim about all or most gorillas.

Can the word some also mean most? The short answer is yes. For example, if I said:

Some doctors prescribe drug X to cure diabetes.

It follows that at least one doctor has prescribed drug X to cure diabetes. But it could be the case that most or even every doctor prescribes drug X to cure diabetes. The definition at least one doesn’t prevent most or all, it just fails to lend them any support. 

Identify the conclusion, main point, or role of a sentence/claim

These questions are called objective questions, meaning that you are not asked to evaluate the argument, but instead to simply examine its structure. Thus you should not look for flaws or errors in reasoning, but instead simply look for the conclusion, support, and determine (if asked) the role of a claim or statement. Examples of the wording of these questions include:

“Which of the following is the main conclusion of the argument?”

“The sentence/statement __________ plays which of the following roles in the argument?”

“The main point of the argument is to”

The more difficult questions in this type are arguments that involve multiple conclusions, sometimes called intermediate conclusions. An intermediate conclusion is a statement that is supported by evidence in the argument, which is used to support the main or primary conclusion of the argument. It can often be difficult to determine which conclusion is the main conclusion and which is used as its support. However, once you have identified all of the conclusions in the argument, you should ask which conclusion can be used to support the other. For example, if I said:

All of the trees in the park will bloom this spring. Last spring, all but one of the trees bloomed. That tree was cut down last week. Thus that tree will not bloom this year in the park.

Notice that this argument has two conclusions: i) all of the trees in the park will bloom this spring, and ii) that tree will not bloom this year in the park. Notice that the first conclusion does not support the second, but the second supports the first. Thus ii) is an intermediate conclusion used to support i), which is the main conclusion of the argument.

Parallel Reasoning 

Questions that involve parallel reasoning come in two forms i) objective reasoning and ii) subjective reasoning. The first is type, objective reasoning, is worded: 

“The reasoning in which of the following arguments most closely resembles the reasoning in the argument above?”

“Which of the following arguments most closely matches the reasoning given in the argument above?”

These questions do not involve flawed reasoning, and merely ask you to evaluate the given argument and match it closely with another argument in the answer choices. It can be helpful to symbolize these arguments. For example, if I said:

When it rains, there is usually lightening. When there is lightening, you shouldn’t play tennis. Therefore, when it rains you usually shouldn’t play tennis.

this argument could be symbolized by showing: 

                                                            x → usually y

                                                            y → shouldn’t z

                                                            x → usually shouldn’t z

Notice that this argument clearly follows and does not contain a flaw. The next step is then to analyze the arguments given in the answer choices, eliminate those that clearly don’t match the form, and then symbolizing the remaining choices. 

A quick way to eliminate wrong answer choices in parallel reasoning is to check for two basic criteria: i) inductive vs. deductive and ii) positive vs. normative. Remember that a deductive argument is one in which the conclusion does not contain a probability. For example, take the following two arguments: 

Math students who study hard usually make good grades.

Math students who study hard probably make good grades.

The first argument makes a definitive claim (i.e. it states something that doesn’t involve a probability). This argument is called deductive. The second argument includes a probability. It is not known whether math students who study hard generally make good grades, it is only known that it is probably the case that they do. This argument fails to make a definitive claim, and thus is called inductive. The arguments on the parallel reasoning questions must match exactly the form they are given in the stimulus. For example, if I gave the argument:

Most ships are probably steered by their captains.

People who steer ships have better eyesight on average.

Thus there are probably some captains with better eyesight than average.

I would note that the word probably appears in one piece of evidence as well as the conclusion. Thus in my parallel argument, I would expect the same to occur.

The second criteria for eliminating answers quickly involves positive and normative arguments. Remember that a positive argument makes a claim about the way the world is and a normative argument makes a claim about the way the world should or ought to be. For example, examine the following two arguments: 

Wendy stole a bike from the store.

Stealing is wrong in most instances.

Thus Wendy likely did something wrong when she stole the bike.

 

Wendy stole a bike from the store.

Stealing is wrong in most instances.

Thus Wendy likely should not have stolen the bike. 

Which one of these is positive and which one is normative? At first it may seem that both are normative. But examine the first argument’s conclusion. Given that we have stated what Wendy did is generally wrong, it follows that it is likely what she did was wrong as well. This is an inductive positive argument. We are not stating that Wendy should or should not have stolen the bike (as it is not clear that she should always do what is right). However, the second argument does make this claim, and while it is supported, it cannot be called positive. Note that if the argument in the stimulus makes a normative claim (i.e. a claim about what should or ought to be the case), the correct answer choice must also make a normative claim. Similarly, if the argument in the stimulus makes a positive claim, the correct answer choice must as well.

Generally, you will find about half of all parallel reasoning questions do not involve flaws (i.e. they have sound reasoning). However, when there is a flaw in a parallel reasoning question, the question will clearly state there is one (or more). Examples of the wording include:

“Which of the following arguments exhibits a flawed pattern of reasoning similar to the argument above?”

“Which of the following arguments contains a flawed pattern of reasoning similar to the argument above?”


“Which of the following arguments most closely resembles the flawed pattern of  reasoning in the argument above?”

Some common flaw questions include the use of the words most and some. Other common words or terms include: almost certainly, usually, generally, unless, and or. Notice the last word or. Or takes on a special meaning on the LSAT, as it is not exclusive. For example, if I said:

I want chicken or beef for dinner.

this does not exclude me wanting both. It could be the case that I want both chick and beef. Thus or is used differently on the LSAT than it is often used in common language. Let’s examine a flawed parallel reasoning that uses language similar to that above:

Drivers of red cars usually exceed the speed limit or forget to fasten their seatbelt.

Michael generally does not speed in his red car, unless he is on the interstate.

Thus Michael sometimes forgets to fasten his seatbelt.

Let’s examine the flaws in this argument. While we know that drivers of red cars usually do one of two things, we don’t know which they usually do! Read the first line very closely. It states that drivers either i) usually exceed the speed limit or ii) usually forget to fasten their seatbelt. Could it be the case that these drivers never fasten their seatbelt, but always go the speed limit? Or vice-verse? Absolutely. The word usually, when it is applied to this sentence, should visually look similar to:

usually [x or y] 

It could be that x usually occurs or y usually occurs. Or both. Let’s move on to the next sentence. Notice the word unless. Again, in this context unless means if not. So if Michael is not on the interstate, then he generally doesn’t speed. But do we know how often he is on the interstate? Maybe he is on the interstate quite often. This statement fails to give us enough information to make any meaningful inferences. Let’s move on to the conclusion. The conclusion has several flaws. First, even if we do assume that Michael generally doesn’t speed, it is not supported that Michael then forgets to fasten his seatbelt. Remember that we don’t know which of those two affects (seatbelt and speeding) usually occur. It could be that generally drivers of red cars speed, but do remember to fasten their seatbelt. If that were true, this claim wouldn’t make any sense. Second, remember that the first sentence makes a claim about what is usually the case. Also notice that the conclusion is definitive (i.e. it states something definite about Michael). But what if Michael is the exception to the rule? Maybe Michael sets a reminder on his phone to alert him to wear a seatbelt. We can never make a definitive claim about an individual from a probabilistic statement. Remember that the correct answer choice will include both of these flaws, and its conclusion will be definitive.

Explain the discrepancy 

A discrepancy question will usually give you two seemingly irreconcilable statements and ask you to find a scenario that would explain why they both occur. For example, take the following situation:

A local auto-manufacturer has recently doubled production of all cars. However, a lake by the factory has not become more polluted as a result, but instead decreased its levels of harmful toxins by nearly half.

 The question asks you to explain why an increase in production would result in a decrease in pollution. Correct answers generally take one of two forms: i) they include a third factor (sometimes called a “common cause”), or ii) they offer some sort of causal relationship between the two that was otherwise overlooked. Let’s look at an example of each. Take the three following explanations:

  1. Due to higher production, the manufacturer has increased his profits and has thus been able to spend a considerable sum to clean up the lake.
  2. While the factory has produced more pollutants, those pollutants have been of the variety X which cause harmful bacteria and toxins in the lake to die more rapidly.   
  1. The lake has experienced some extra pollution, but not enough to significantly increase its toxic levels. 

The first explanation shows a causal relationship between more production and less pollution. If the manufacturer makes more money, it will spend more to clean up the lake. Notice that this addresses not only both the extra pollution and the increase in production, but how they are related. The second example mentions a third cause (the bacteria). Again, this explanation addresses the way these two phenomena are related, and offers something not accounted for in the stimulus. Let’s examine the third explanation. This explanation fails to show why an increase in pollution would result in a decrease in its toxic levels. While it does state the problem, it does not offer a solution to why it occurred. The most common wrong answer choices on these question types will restate the problem, but fail to offer a reason why it occurred.

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