[Page 1 of 34] Logical Reasoning Intermediate Workshop Welcome to Kaplan's Logical Reasoning Intermediate Workshop! In this workshop, we will introduce several question types and go in depth into arguments. Specifically, this workshop will cover:
A review on the basics of an argument The Logical Reasoning question types o Strengthen/Weaken questions o Inference questions o Logical Flaw questions
Let's start with a brief review on the basics of an argument.
[Page 2 of 34] Review the Basics: Major Definitions Look at each of the terms or principles mentioned below. Jot down-or think through-whatever you know or remember about each, and then roll your mouse over each term to compare your notes with ours. 1. Argument 2. Conclusion 3. Evidence 4. Assumption 5. Topic and Scope 6. Keywords
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Keywords indicating Conclusion Keywords indicating Evidence Keywords indicating Filler 7. One Sentence Test
[Page 3 of 34] Review the Basics, continued To refresh your Reasoning skills, read the argument below and identify the evidence, conclusion, and central assumption. Click Continue when you're done.
Venturian Capital Funds forbids its employees to own any mutual funds. Mr. Wright is a Venturian employee. Therefore, Mr. Wright does not knowingly own any mutual funds.
The first two sentences are the evidence: The company forbids employees from buying mutual funds, and Mr. Wright is an employee at this company. The last sentence is the conclusion: Mr. Wright does not knowingly buy mutual funds. The conclusion is signaled by the Keyword therefore. The central assumption is that Mr. Wright does in fact follow the dictates of his employers. To strengthen this argument you would try to confirm the central assumption, e.g., "Mr. Wright studiously follows all of the rules of his employer," or "No employee of Venturian violates its rules." To weaken this argument you should try to undermine this assumption, e.g., "Mr. Wright violates the precepts of Venturian Capital whenever he wants," or "Every Venturian Capital employee violates its no-mutual fund rule at some point or another." Before we start a more thorough understanding of the strengthen/weaken issue, let's review the Kaplan Method for Logical Reasoning. The Kaplan Method for Logical Reasoning Hopefully, by now, you are very familiar with the method. To review, try this: Type Step 1 of the Kaplan Method into the textbox below, and then click Continue to compare your notes with ours. Repeat for Steps 2, 3, and 4.
Identify the Question Type
Identify the question stem: Use it to determine what you are looking for in the stimulus.
2 Untangle the Stimulus Untangle the Stimulus: Study it carefully. Find evidence and conclusion when you can, and paraphrase the whole thing in simple terms.
Think Critically about the Answer
Thinking critically about the right answer before looking at the answer choices is a hallmark of every good test taker. With some questions in the Logical Reasoning sections you will be able to make a prediction of the answer. On
others, however, it is better to go directly to the answer choices.
Evaluate the Choices
Evaluate the Choices—either find one that matches your prediction, or go through the choices looking confidently for the one and only one that answers the question. Keep using and reviewing the Kaplan Method for every Logical Reasoning question. [Page 5 of 34] Question Type: Strengthen/Weaken the Argument What does strengthening or weakening an argument mean? The concept goes back to the basic idea of an argument. Remember, an author uses an argument to persuade someone to accept his/her view. The author is stating a conclusion and using evidence to verify this conclusion. S/he is moving the evidence to the conclusion—a movement that is fraught with opportunities for errors. Like we said in the Logical Reasoning Basics workshop, the majority of the arguments in the Logical Reasoning sections are weak. They contain flaws or they require assumptions to tie the evidence to the conclusion. The Strengthen/Weaken question type specifically tests our ability to understand the strength or weakness of an argument. Strengthening an argument means to cement the connection between evidence and conclusion. In other words, it means to make the conclusion more likely to follow from the evidence. A valid strengthener does not need to prove an argument—it only has to make the conclusion more likely to occur. [Page 6 of 34] Strengthen the Argument Remember the argument about writers for Clarion? The editor says that I am the best writer on the Clarion's staff. But I disagree— Mary is; she writes more articles than all the rest of us put together. How can we make it more likely that Mary is the best writer on the Clarion's staff? On the LSAT, strengthening an argument is primarily done by affirming the assumption. How might you strengthen this argument? Type your answer in the textbox and then click Continue to compare your answer with ours.
Confirm that the best writer is the one that writes the most articles.
If it is true that the best writer is determined by the quantity of articles written, then Mary is definitely the best writer. Quite frequently, the correct answer to a Strengthen question will be a statement that simply restates or shores up the central assumption. Now consider the next argument. How might you strengthen the spokesman's claim? Type your answer in the textbox and then click Continue to compare your answer with ours. Thousands who suffer heart attacks each year die before reaching a hospital or clinic where medical professionals can administer drugs which dissolve clots found in coronary arteries. The Food and Drug Administration has approved a new blood clot dissolving agent which, a spokesman claims, can save the lives of many people who would otherwise join this group of heart attack victims. Strengthen:
Confirm that the drug can be self-administered, or in an ambulance en route to a hospital or clinic.
The spokesman is assuming that the "new blood clot dissolving drug" can be administered prior to an arrival at a hospital or clinic. If we strengthen this assumption, we would make it more likely that "many people," who would not have made it to a hospital or clinic, will be saved. Let's try an in-format Strengthen question. [Page 7 of 34] Managing Strengthen Questions What you will see in Strengthen questions are question stems like these: Which of the following, if true, most strengthens the argument? Which of the following, if true, could provide logical support for the thesis above? The older idea would regain acceptance upon the declaration that
The argument would be more persuasive if which of the following were found to be true? You should then think: "Strengthen: The right answer makes the conclusion more likely to follow from the evidence." Next, what you should do is identify evidence and conclusion and predict what would make it more believable that evidence leads to conclusion. Look to affirm the central assumption. [Page 8 of 34] Strengthen Questions: An Example Try this sample question. Make sure to use the Kaplan Method for Logical Reasoning and click Continue when you have chosen your answer. Top of Form Dr. Anderman's years in a prison had little influence on her writing. A comparison of Anderman's pre-incarceration novels with the short stories she published in the three years following her release demonstrates a remarkable consistency of themes, symbolism, and style. Which of the following, if true, most strengthens the conclusion drawn above? (A) Dr. Anderman wrote nothing during her imprisonment. (B) Novels and short stories, even when written by the same author, are distinctly different literary forms. (C) Dr. Anderman herself discouraged consideration of personal issues in any stylistic analysis of her writing (D) The short stories were written subsequent to Dr. Anderman's imprisonment. (E) Dr. Anderman's writing concentrates primarily on themes of personal, rather than societal, conflict. Bottom of Form
The One Sentence Test reveals that the first sentence is the conclusion It's the author's point that the doctor's prison experience didn't change her writing. The evidence is the similarity of the pre- and post-imprisonment work. But IS it pre- and post-? True, the short stories were "published" after she was released, but when were they "written"? If they were composed before the POW experience, then the author can not use them as an example of a "consistency of themes, symbolism, and style" and hence, nor could the
author draw the conclusion. In short, the author is assuming that the short stories, published after her release, were also written after her release, and (D), by confirming that assumption, stands as the strengthener. Strengthening and weakening are different sides of the same coin. Let's see how. [Page 9 of 34] Weakening the Argument Weakening an argument is not the same thing as disproving it. Weakening means to make the conclusion less likely to follow from the evidence. On the LSAT, an argument can be weakened by undermining the evidence, conclusion or assumption. Here's the Clarion argument again: The editor says that I am the best writer on the Clarion's staff. But I disagree— Mary is; she writes more articles than all the rest of us put together. How might you weaken the argument? Type your answer in the textbox and then click Continue to compare your answer with ours.
Show that quantity does not translate to quality.
This statement undermines the central assumption, and weakens the author's argument that because Mary writes the most articles, then she must be the best writer. If quantity does not mean quality, then the author can not conclude the most prolific writer is the best writer. And now, how might you weaken the spokesman's claim? Type your answer in the textbox and then click Continue to compare your answer with ours. Thousands who suffer heart attacks each year die before reaching a hospital or clinic where medical professionals can administer drugs which dissolve clots found in coronary arteries. The Food and Drug Administration has approved a new blood clot dissolving agent which, a spokesman claims, can save the lives of many people who would otherwise join this group of heart attack victims.
Confirm that administering the drug requires a medical professional at a medical institution.
This new drug will only live up to its billing if it can be administered "before [one reaches] a hospital or clinic." The claim is weakened if that's proved false. Let's try an in-format Weaken question. [Page 10 of 34] Managing Weaken Questions What you will see in Weaken questions are question stems like these: Which of the following, if true, most weakens the argument? Which of the following, if true, would provide support against the thesis above? Which one of the following statements, if true, is a valid criticism of the above argument?[ The claim in the commercial would be most seriously undermined if which of the following were found to be true? You should then think: "Weaken: The right answer makes the conclusion less likely to follow from the evidence." Next, what you should do is identify evidence and conclusion. Then, predict what would make it less believable that evidence leads to conclusion. Look for alternatives and assumptions.
[Page 11 of 34] Weaken Questions: An Example
Try this sample question. Click the circle by the correct answer choice, and then click Continue. Remember to use the Kaplan Method for Logical Reasoning. Top of Form An investigation must be launched into the operations of the private group that is training recruits to fight against the Balaland Republic. The United States Neutrality Act plainly forbids United States citizens from engaging in military campaigns against any nation with which we are not at war. Since no war has been declared between the United States and the Balaland Republic, we should bring charges against these fanatics, who are in open defiance of the law. Which of the following, if true, would most weaken the argument above? (A) The recruits are being trained to fight only in the event the United States goes to war against the Balaland Republic. (B) Diplomatic relations between the United States and the Balaland Republic were severed last year.
(C) The Balaland Republic is currently engaged in a bloody and escalating civil war. (D) The training of recruits is funded not by United States citizens, but rather by a consortium of individuals from abroad. (E) Charges cannot be brought against the private group that is training the recruits unless an investigation is first launched. Bottom of Form
The stimulus tells us that U.S. law forbids U.S. citizens from engaging in military campaigns against countries unless the United States is at war with those countries. Since no war has been declared between the United States and the Balaland Republic, the author concludes that the recruits being trained to fight against the Balaland Republic are defying U.S. law. But if, as Choice (A) asserts, the recruits are being trained to fight only if a war is declared, then they're not in defiance of U.S. law; this weakens the author's conclusion. Preparing for battle is different from actually engaging in it.
[Page 12 of 34] Strengthen/Weaken: One Final Point To strengthen an argument does not mean that you have proved it, just as to weaken an argument does not mean that you have disproved it. Strengthening and weakening have to do with the link between the evidence and conclusion. We want to make that link stronger or weaker.
Try this sample question, click continue after you have selected your answer, and read the explanation carefully. Remember to use the Kaplan Method for Logical Reasoning.
Top of Form
Attendance at the games of Forrest City's minor league baseball team is down and the team needs our support. Ever since a soccer franchise was established here, the local newspaper, the Constitution-Herald, has reduced its coverage of baseball. Therefore, the presence of the new soccer franchise is hurting the attendance at the minor league team's baseball games. Which of the following, if true, would most weaken the argument above? (A) The Forrest city soccer franchise includes two highly popular players. (B) All minor league baseball teams are suffering reduced attendance. (C) The owner of the local baseball franchise also owns the soccer franchise. (D) Nearly all local fans who go to baseball games read sports coverage only in the Forrest City Sports Fan, a local magazine. (E) In general, newspapers readers in Forest City prefer to watch basketball.
Bottom of Form
The word "Therefore,…" tells us that the conclusion is that the soccer team is hurting the baseball team, the evidence is that since the soccer team's arrival, the coverage of baseball in the Constitution-Herald has declined. The author is assuming a causal link between the coverage in the newspaper and baseball attendance. So to weaken the argument, we need to provide evidence countering that link, and (D) provides it. Now here's the point: If (D) is true, then it is still possible that the new soccer franchise is hurting baseball attendance—but not for the reason proposed. There would have to be some other reason why baseball attendance is dropping because of soccer: Maybe people in Forrest City are spending so much money going to soccer games that they can't afford to go to baseball games as well. (Note that the following statement would strengthen the argument: "The disposable income of the average sports fan in Forrest City cannot support attendance at two sports.") But if (D) is true, then the proposed evidence is no longer relevant to the author's conclusion. This is what weakening means. Now let's look at some skills you will need to learn to tackle another common question type: Inference questions.
[Page 13 of 34] Introduction to Inference In the last workshop and in the beginning of this workshop we were dealing exclusively with arguments. The vast majority of Logical Reasoning questions are based on arguments. But there are exceptions, and the Inference question is the main exception you will see on the LSATs.
Read the following passage and consider its structure carefully; then click Continue.
In 1996, 84% of political donations from businesses came from those with annual sales of over 100 million dollars. In that same period, 24 companies in the software industry, all of which had shown increases in the valuation of their stock of over 40% in the preceding year , accounted for 19% of business political donations. Furthermore, pharmaceutical companies accounted for 9% of all political campaign contributions in 1996.
Do you see the difference? The author is not trying to fulfill an argumentative purpose, but rather, s/he has assembled a set of facts on a topic. Whenever you see a set of facts, the test-maker will be, in almost every case, asking you an Inference question.
[Page 14 of 34] Definition of Inference "Inference" has a different meaning in everyday life from its meaning on standardized tests. In everyday life, the word inference is usually a synonym for educated guess. In other words, when we make an inference in everyday life we are deciding what is probably true based on the given information. For example, suppose you are walking along the street and a friend runs out of the house in tears. You might make the inference that "My friend is upset. Something terrible has happened." Would your inference have to be true? It could be true but by no means must be. Maybe your friend's eyes were irritated. Maybe your friend just won the $1,000,000 Lottery or maybe your friend just received a marriage proposal. Any of those outcomes is possible. But, you made an inference ("My friend is upset") based on the best possible information, and that is the way inferences work in everyday life. We make inferences—whether or not they prove to be hard facts—based on the best information available to us. But not on standardized tests. In the LSAT Logical Reasoning section, an inference is a valid deduction based on the stimulus. In other words, an inference is a statement that must be true if everything in the stimulus is true. In other words, when it comes to the Logical Reasoning section, the word inference means "that which must be true." An "educated guess" is not good enough. Only, one of the answer choices is credited, and the other four are categorically
[Page 15 of 34] Definition of Inference, continued Let's look at a simple example. Study this slightly modified version of our Clarion writers argument. The editor of the Clarion staff has decided that the best writer is the one who writes the most articles. Mary writes more articles than all the writers at Clarion put together.
If the above statements are true, what can you infer from this argument? Click Continue to compare your answer to ours.
We can infer that Mary is the best writer on the Clarion staff. This statement, like all correct answers to Inference questions, must be true if the information given in the passage is true.
[Page 16 of 34] Inference Question Types There are two types of inference questions on the LSAT
One type gives a set of statements and asks the test taker to make a deduction of the conclusion. The second type is a deduction based on a single statement or a combination of statements.
Let's see how the first type of Inference question works. [Page 17 of 34] Managing Inference Questions An important thing to remember for all inference questions is that the wording for Inference questions is the most varied of all the Reasoning question stems. Sometimes the stem is direct, and you can easily tell that you're seeing an Inference question. Sometimes the stem wording is subtle, and sometimes it looks like it belongs to another question type entirely. What you might see in the Inference question type are question stems like these: Which one of the following is probably the conclusion toward which the author is moving? Which one of the following can be properly concluded from the information given above? If the statements above are true, then which one of the following must also be true? Which one of the following, if true, best explains how the statements above can both be true?
Which one of the following can be most reasonably inferred from the statements above? The author suggests Which one of the following is implied by the argument above? You should then think: "An inference is something that must be true based on one sentence or a combination of sentences." Next, what you should do is understand each sentence individually, noting connections between them, and evaluate each choice. It is difficult to predict an answer for an inference question so if you can not think of a prediction, move directly into the choices. [Page 18 of 34] Inference Questions: Example 1 Try this sample question. Click the circle by the correct answer choice, and then click Continue. Remember to use the Kaplan Method for Logical Reasoning questions. June has spent the last two year attempting to define the function of the newly discovered kap-2 protein. She believes it is involved in the replication origin. Despite all her efforts, her experiments have consistently failed. An essential step in the experiment involves depleting her extract with the antibody of the kap-2 protein. However, the depletion destroys the extract. She speculates that her kap-2 antibody is either depleting other essential replication proteins or the antibody's excessive IGg concentration is masking these proteins. Both of these problems are not solvable with present biochemical procedures. Which one of the following is probably the conclusion toward which the author is moving? (A) June will not be able to discover the role of the kap-2 protein. (B) The masking of the essential proteins must be solved in order for June to define the function of her protein (C) The kap-2 protein is not involved with the replication origin. (D) June will not be able to move forward in her experimental research unless she designs a new experiment or discovers a solution for the depletion problem. (E) The homology between the kap-2 protein and other replication proteins is very similar and therefore, the problem is insolvable.
If we cut through all the technical jargon, we can paraphrase the argument as basically: "June wants to find what this kap-2 protein does, but her experiments have failed. Why? She speculates a couple reasons; both of which can not be
solved with present biochemical procedures." If the author had to sum up the argument in one sentence what would it be? Well, it would be choice (D). June will not find the function of kap-2 unless she designs a new experiment or she discovers a solution to the depletion problem. (A) is to too extreme. Just because she has failed so far, does not mean she will fail in the future. (B) says the masking of the protein must be solved. However, she could solve the problem by designing a new experiment without using the kap-2 antibody for the depletion. Moreover, the argument does not say that the "masking" is definitely the problem. (C) once again is too extreme. (E) talks of the similarity (homology) of the kap-2 protein with other replication proteins and therefore, says the problem is not solvable. First, this answer choice is outside the scope. Second, the argument never says the problem is not solvable, it is just not solvable with present biochemical procedures. It is once again too extreme. (We will spend more time talking about extreme answers in the next workshop, for now just keep it in mind.) [Page 19 of 34] Inference Questions: Example 2 The correct answer to our second type of Inference question does not need to reflect every sentence in the text. In fact, it's more likely that the right answer either rewrites one sentence, or combines two sentences. Let's try one. You've already read the following set of facts. Read it again, and then answer the question that follows. Remember to use the Kaplan Method for Logical Reasoning. Top of Form In 1996, 84% of political donations from businesses came from those with annual sales of over 100 million dollars. In that same period, 24 companies in the software industry, all of which had shown increases in the valuation of their stock of over 40% in the preceding year, accounted for 19% of political donations from businesses. Furthermore, pharmaceutical companies accounted for 9% of all political campaign contributions in 1996. If the above statements are true, which of the following must also be true? (A) In 1996, computer companies contributed more to political campaigns than did pharmaceutical companies. (B) At least one of the businesses with annual sales of over 100 million dollars showed an increase in the value of its stock in 1995. (C) Most software companies had sales of less than 100 million dollars in 1995.
(D) No software company that made political contributions in 1996 had sales of over 100 million dollars in that year. (E) The stock of most businesses with annual sales over 100 million dollars did not increase by over 40% in 1996. Bottom of Form
(B) is the right answer—and notice that it is inferable based on sentences 1 and 2 only. We know that of the total pot of all political contributions by businesses, 84% came from businesses with over $100 million in annual sales, while 19% came from a group of 24 software companies, each of which showed an increase in its stock value. Since 84% + 19% > 100%, there must have been some overlap between the two groups of businesses. Therefore, at least one of the 24 software companies also had annual sales of $100 million. Since this company also showed an increase in its stock value in 1995 (the previous year), at least one of the companies with annual sales of $100 million or more is a software company with an increase in its stock value. That's (B), and the final sentence has nothing to do with it. Choice (A) is tempting, except that we know that a set of software companies contributed 19% of contributions by businesses whereas pharmaceuticals made contributions totaling 9% of all contributions. If the total of all contributions is much larger than that portion of contributions coming from businesses, it is possible that 9% of the total is more money that 19% (or more) of the subset of contributions coming from businesses. As for (C), we do not have any info on most software companies, just a group of 24 of them, so we have no way of knowing. Choice (D) is contradicted by the text. As correct choice (B) asserts, there must be at least one company that: sells software, sold over $100 million, and contributed to campaigns. Finally, choice (E) concerns companies we do not know anything about. We have no information about the stock performance of those companies with more than $100 million in annual sales that made political donations, much less about all the companies in that class that made no contributions, so (E) is outside the scope. Let's learn about formal logic in Inference questions.
[Page 20 of 34] Inference Questions: Formal Logic Some Inference questions test your basic formal logic skills. In these questions, you should identify, translate and if you can, combine formal logic statements found in the stimulus. You will cover formal logic more in depth during your lessons and in other workshops. For now, let's cover the essential basics. The basic formal logic statement is the if-then statement: If X, then Y. This statement is the logical equivalent of the following statements: All X are Y
Only Y are X X only if Y No X unless Y If Not Y, then Not X In other words, any of the above statements can be translated to the statement "If X, then Y" and vice versa. Another important formal logic point is: in the statement "if X, then Y," X is the sufficient statement and Y is the necessary statement. For example, in the statement, "If you are an canary, then you are a bird." Being a canary is sufficient for being a bird and being a bird is necessary for being a canary. Let's try a question.
[Page 21 of 34] Inference Questions: A Formal Logic Example Try this sample question. Click the circle by the correct answer choice, and then click Continue. Remember to use the Kaplan Method for Logical Reasoning. Top of Form If you are a poet, then you are an artist. All painters have automobiles. Some painters are artists. If all of the statements are true, which of the following must also be true? (A) Some poets have automobiles. (B) Some artists have automobiles. (C) Some painters are poets. (D) No artists have automobiles. (E) All poets have automobiles. Bottom of Form
The first statement tells us that all poets are artists. This does NOT mean all artists are poets. All you know is that if you are a poet you are an artist. You can translate the first statement into its contrapositive: "If you are not an artist, then you are not a poet." The second statement says if you are a painter then you have an automobile. Once again, you can NOT translate this to mean: "If you have an automobile, then you are an artist," but you can translate it into its contrapositive: "If do not have an automobile, then you are not a painter." The last line states "some painters are artists." In formal logic "some" means "at
least one." It does not mean: a few, many, most, or all. Now since some painters are artists and all painters have automobiles. Then the artists that are painters have automobiles or some artists have automobiles, choice (B). Choices (A), (C) and (D) are all guesses. There is no overlap between the subjects of each choice. (D) is the 180 choice (we will talk more about common wrong answer choices in the Challenge workshop). It is the exact opposite from the correct answer.
[Page 22 of 34] Inference Wrap-Up If you're having trouble with an Inference question type, remember these key principles:
The correct answer to an Inference question must be true based on the given information. An inference need not come out of every sentence of a passage. Sometimes it's simply a rewrite of one sentence, or a deduction based on the combining of two sentences. An inference stays in line with the author's gist, scope, tone and point-ofview. An inference is neither denied by, nor irrelevant to, the argument. The wrong answers in Inference questions either depart from the scope of the text, go too far, or contradict the text
There are four common types of questions in the Reasoning section and they account for 70% of the questions in this section. We have seen three types: Assumption, Strengthen/Weaken and Inference and on the next page we will introduce the fourth: Logical Flaw questions.
[Page 23 of 34] Logical Flaws What do Logical Flaw questions test? Remember our discussion on arguments? The author creates an argument by using evidence to validate his or her conclusion. As we said before, there are many possibilities for errors when moving from evidence to conclusion and, on the LSAT, many of these errors are tested. In Strengthen/Weaken questions your task was to discover that a flaw exists and to use the flaw to strengthen/weaken the argument. The Logical Flaw questions, on the other hand, specifically test your ability to identify the flaw created by the author of the argument. The LSAT tests several common flaws over and over again including:
Alternative Possibility Causation vs. Correlation
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Necessity vs. Sufficiency Number vs. Percent Scope Shift Flaw of Representativeness Unwarranted Assumption The more familiar you are with the flaws, the quicker you will be able to identify them and consequently, the better you will do on the LSAT. Let's briefly discuss each of the common flaws.
[Page 24 of 34] Common Logical Flaws: Alternative Possibilities Arguments created by writers of standardized tests are in many cases structured in noticeable patterns. In fact, many arguments are just variations on the same basic themes. One of the most prevalent of these "classic arguments" is the causal argument. In a causal argument, the author argues that X caused Y. The author is attempting to prove that a particular effect is the result of a particular cause. Causal arguments are not always faulty (have logical flaws), however, they often are on the LSAT. In every causal argument there is the underlying assumption that no other cause could have been responsible for the observed effect. A common flaw is the failure to consider these alternative possibilities. Let's look at an example. Consider the following statements and find the flaw. Jane takes the train to work everyday. However, today she is late to work. She must have missed her train this morning.
What's wrong here? The author is assuming that Jane's tardiness is due to her missing the train. She does not consider that there are numerous alternative possibilities for Jane's late arrival to work.
[Page 25 of 34] Common Logical Flaws: Correlation vs. Causation The "Correlation vs. Causation" flaw is also a common flaw of the causal argument. Remember, a causal argument claims that one event triggered another event. A possible flaw is neither event triggered the other, instead they just occurred simultaneously. The events only correlated with each other. If A and B appear together, A and B are correlated with each other. Causation is
far more specific-either A must cause B, or B must cause A. Read the following statement. What is wrong with the argument? Click continue to see an explanation. Whenever the TV networks start broadcasting many reruns, beach attendance increases. Reruns, therefore, must cause people to flock to the beaches.
The author assumes that because reruns and high beach attendance are correlated, reruns must have triggered people to go to the beach. However, the author never gives evidence that reruns caused people to go to the beach. The flaw is the author is assuming causation when there is only a correlation between the two events.
[Page 26 of 34] Common Logical Flaws: Necessity vs. Sufficiency We saw "necessity vs. sufficiency" during our brief formal logic review. Arguments in the Reasoning section can be flawed because they confuse necessary conditions for sufficient conditions. Consider the following statements and then choose an answer. Click Continue when you are done. Top of Form In order to win the World Series this year a team must have good pitching. The New Jersey Robins have good pitching. Will the Robins definitely win the World Series? Yes No Bottom of Form
The Robins have good pitching, but this does not guarantee that they will win the World Series. It is just a necessary factor for them to win, however, there could be many other factors that the Robins may not possess. If good pitching was sufficient to win the World Series, then the Robins would, in that case, "definitely win the World Series."
Top of Form In order to win the World Series, it is enough to have the two best pitchers in the League. The New Orleans Pelicans have the two best pitchers in the league. Will the Pelicans definitely win the World Series? Yes No Bottom of Form
Do you see the difference? In this situation, it is sufficient to win the World Series, if the team has "the two best pitchers." Once the requirement is met, the result is guaranteed. [Page 27 of 34] Common Logical Flaws: Number vs. Percentage Whenever you see an argument with numbers and percentages, ask yourself: is the author making an inaccurate conclusion based on a confusion between numbers and percentages? Read the following statement. What is wrong with the argument? Click continue to see an explanation. Newfoundlands have a high probability of developing hip dysphasia. In contrast, there is a very small percentage of Poodles with hip dysplasia. Therefore, there must be more Newfoundlands with hip dysplasia then Poodles with hip dysphasia.
What's wrong with this argument? Let's say there are 100 Newfoundlands in the world and all 100 have hip dysplasia. Let's also say that there are 10,000 Poodles and only 10% have hip dysplasia. That would make 1,000 Poodles with hip dysplasia and consequently, more poodles with hip dysplasia than Newfoundlands. See how the author commits a flaw by confusing percentages with absolute numbers.
[Page 28 of 34] Common Logical Flaws: Scope Shifts The scope as you may recall from the Basics workshop, is the way the author narrows down the topic. In LSAT arguments the scope is usually a combination
of evidence and conclusion. When the subject of the evidence and the subject of the conclusion are not the same, this is the flaw of shifting the scope. In scope shift arguments, the author cites evidence about one subject, and then subtly shifts the scope to make a conclusion about a different subject. Read the following statement. What is wrong with the argument? Click continue to see an explanation. Principal: Ten teachers called in sick today and all the substitutes are teaching at the school in the next town. We are going to have to cancel ten classes.
Do you see the scope shift. The evidence deals with teachers and the conclusion deals with classes. What if the ten teachers taught thirty classes, well the principal would then have to cancel thirty classes. Or what if, in this school district, two teachers taught each class. Well, in that case the principal would only have to cancel five classes. See how this argument makes the flaw of a scope shift?
[Page 29 of 34] Common Logical Flaws: Flaw of Representativeness A variation on the scope shift flaw is the representativeness flaw. In the flaw of representativeness, the author draws a conclusion from evidence from a different sample. The key feature in all representativeness arguments is that the author assumes that sample used in the evidence is sufficiently representative to draw broader conclusions from it. Read the following statement. What is wrong with the argument? Click continue to see an explanation. The school principal wanted to determine whether there was enough demand for the school cafeteria to provide a salad bar. He surveyed student council members and found a majority wanted the salad bar. As a result, the principal decided to have the salad bar installed.
The principal based his decision on a survey of student council members. But are they representative of the dietary tastes of the students at the school in general? Maybe or maybe not. The principal, however, assumes that the student council members surveyed are, in fact, representative. If she is right, the fact the salad bar is popular with student council members means that they will be popular with the student body. However, the principal can not assume this
without further evidence. As we said, this is the central underlying assumption in a representativeness argument: that the sample used as evidence is representative of the larger group.
[Page 30 of 34] Common Logical Flaws: Flaw of Representativeness A variation on the scope shift flaw is the representativeness flaw. In the flaw of representativeness, the author draws a conclusion from evidence from a different sample. The key feature in all representativeness arguments is that the author assumes that sample used in the evidence is sufficiently representative to draw broader conclusions from it. Read the following statement. What is wrong with the argument? Click continue to see an explanation. The school principal wanted to determine whether there was enough demand for the school cafeteria to provide a salad bar. He surveyed student council members and found a majority wanted the salad bar. As a result, the principal decided to have the salad bar installed.
The unwarranted assumption is that each teacher teaches one class. If you make this assumption, then you can properly conclude that ten classes will be cancelled.
[Page 31 of 34] Managing Logical Flaw Questions What you will see in the Logical Flaw questions are question stems like these: The argument is flawed because The reasoning in the argument is flawed because the argument Which one of the following is an error in reasoning committed by the argument? The politician's reply is most vulnerable to which one of the following criticisms. You should then think: "A Flaw question: Something has gone wrong between evidence and conclusion, and chances are I have seen this form of bad logic before." Next, what you should do is find the evidence and conclusion and be on the lookout for disconnects between evidence and conclusion. Then, predict the flaw. Now that we covered many of the common flaws, let's try a question.
[Page 32 of 34] Logical Flaws: An Example Top of Form Historians frequently argue that an outlet for population overflow is required for a country's economy to prosper. But we need look no further than our own shores to find counter-evidence: The nation of DeSouza has long been able to rid itself of its surplus population by sending people to the U.S., and yet its economy has done quite poorly. The reasoning above is most vulnerable to which one of the following criticisms? (A) It mistakenly interprets the historians to be claiming that a factor guarantees, rather than is necessary for, a result. (B) It relies on evidence that merely restates the argument's conclusion. (C) It uses an analogy that ignores an important distinction between the things being compared. (D) It attacks a view by calling into question the character of the supporters of that view. (E) It presents an argument without offering any evidence in support. Bottom of Form
Choice (A) is correct. The DeSouza example—of a country that has ended its population surplus and still is economically weak—is a poor counterexample because historians aren't saying that a population outlet is sufficient for economic health, just that it is necessary for health. (A proper counterexample would cite a nation that had no surplus-population outlet and nevertheless was prospering.) Choice (A) describes the flaw as a necessity vs. sufficiency flaw.
[Page 33 of 34] Review the Topics Strengthen/Weaken question type: In a strengthening/weakening question your task is to affirm/undermine the link between the evidence and conclusion. In this question type look for alternative possibilities and always look for the underlining assumption. Inference Questions: In both inference questions, your job is to take the facts and make a logical deduction. In the Main Point question type, you are specifically looking for the author's conclusion. The point the stimulus is driving towards. In the Inference question type, you are looking for a logical deduction from the
given facts. Many times it may only involve one or two sentences from the stimulus. In some questions, you may have to use your knowledge of formal logic to find a logical deduction. Logical Flaw question type: In this question type you are asked to find the flaw that the author has committed. We reviewed some common flaws including:
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Alternative Possibilities Causation vs. Correlation Necessity vs. Sufficiency Number vs. Percent Scope Shift Flaw of Representativeness Unwarranted Assumption
The Assumption, Strengthen/Weaken, Inference and Logical Flaw question types account for 70% of the questions in the Reasoning section. They are the foundation of your score, so make sure to practice them.
[Page 34 of 34] The Logical Reasoning Intermediate Workshop: Next Steps Once you feel comfortable with the concepts in this lesson, take the Logical Reasoning Intermediate Workshop Quiz. This will give you a chance to practice what you have learned and will give you feedback after you are done. If you need an additional review at any time, return to this workshop. Have fun and good luck!
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