SDCAR2010【逻辑入门】（十）Flaw (part 2)
Common flaws (continued)
9. Overstating the evidence
• No evidence for a claim does not mean the claim is false.
• No evidence against a claim does not mean the claim is true.
• Some evidence for a claim does not mean it is true.
• Some evidence against a claim does not mean it is false.
10. Survey errors
A) Biased or unrepresentative samples
• When studying a certain groups, researchers must survey a random sample of that group or the survey results will be flawed.
• Researchers can err by choosing people who are likely to represent only a subset of the entire group or by letting people voluntarily respond. Either way, the sample is not random and thus unrepresentative.
• Example: Randomly polling subway riders all across the country to determine whom Americans will likely elect as President will probably not account for rural voters, who tend to vote differently than urban voters.
B) Biased questions
• Tempting: “Do you support reasonable reform?” encourages respondents to say they do because to say otherwise would be “unreasonable.”
• Misleading: “Do you think we should kill Osama bin Laden or stop searching for him?” suggests we have only two choices. (Well, now we do not need to worry about this question any more!)
• Confusing: “Do you have a higher elasticity for bread or cheese?” is really asking, “Are you more sensitive to a change in price for bread or cheese?” or, better yet, “Which food prices do you monitor more closely?”
C) Biased answers
• Example: In the 2008 Presidential primaries, polling critics questioned polls that showed Barack Obama leading Hillary Clinton. They argued that the respondents were likely overstating their support for Obama because they did not want to appear racist.
11. Generalizations
• The passage makes an unwarranted generalization based on only one or two examples.
• Flawed example: “Poor people will go from rags to riches. Felix Dennis did.”
• Sound example: “Poor people can go from rags to riches. Felix Dennis did.”
12. Quantity vs. percentage
• When both number and percentage are stated and compared in the same passage, pay extra attention to what can be said in the comparison. Simple math would solve some seemingly complex CR problems.
• Example: “Tom was paid twenty percent more this year than last year, while his administrative assistant, Sally, was paid only ten percent more. Therefore, Tom was paid more this year than Sally was.” What if Tom’s initial salary was less than half of that of Sally?
13. Parts vs. whole
• What is true for the parts might not be true for the whole: “Every part of this car is well built, so this car is well built.”
• What is true for the whole might not be true for the parts: “Apple sells only computers. So every Apple sales representative sells only computers.”
• What is true for some parts might not be true for the other parts: “May cat cleans herself daily. So all cats clean themselves daily.”
14. Past vs. future: “The bus has never been late. It will be on time today.”
15. Using the same word twice in the argument but with different meanings.
16. False choice
• When a passage argues A is the right choice because A and B are the only options and B won’t work, make sure A and B are the only options. If not, the argument relies on a false choice.
17. False analogy
• Analogies are always different than the claim they support. But false analogies are different in some important respect.
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