9Critical Thinking Is there any evidence to support the claim? Is the evidence reliable and trustworthy? How reliable is it? Should you accept it?Does the evidence actually support the claim?Is there other evidence you should consider?
12Wason Selection TaskSuppose that I present you with four cards. On each card there is a number on one side and a color (blue or red) on the other. I claim: If a card has an even number on one side then it is blue on the other side. Which of the four cards do you need to turn over to tell whether this claim is true or false?
20Wason Selection TaskCard D doesn’t matter. Two possibilities: 1. The other side is even (for example, it’s “4”). The claim says if it’s even, then it’s blue. It does not say that if it’s blue, then it’s even.
21ExampleTrue claim: If something is a dog, then it is an animal. Does not mean false claim: if something is an animal, then it is a dog.
22Wason Selection TaskCard D doesn’t matter. Two possibilities: 2. The other side is odd (for example, it’s “3”). The claim says if it’s even, then it’s blue. It does not say that if it’s not even, then it is not blue.
24Wason Selection TaskCard B is important. 4 is an even number. If other side of card B is red, then the claim is false, because B is a card with an even number on one side but it is not blue on the other side. You must turn over B and make sure it is not red on the other side.
26Wason Selection TaskCard C is also important. If the claim is true, this card must have an odd number on the other side. If it has an even number on the other side, then the claim is false. You must turn over #3 and make sure there is not an odd number on the other side.
27Statistical ResultsAround ½ of people studied say “B: 4” and “D: Blue”. About 1/3 say just “B: 4”. Only about 1/20 get the right answer: “B: 4” and “C: Red”!
28AnalysisPeople look for results that would agree with the claim. Turning over B and D, you could get agreement– for example, [4, Blue] and [6, Blue]. You cannot get agreement by turning over C. But you can get disagreement and that is why the card is important!
31Cherry PickingIf you see cherries in the store, you might notice that most or all of them were ripe and healthy. But you couldn’t conclude that most or all cherries are ripe and healthy. These ones have been selectively picked for good condition.
32Fluoride and Oral Health Fluoride is a chemical that is known to prevent tooth-decay. It is in most toothpastes and many places in the world put it in the drinking water. The US CDCP says water fluoridation is “one of 10 great public health achievements of the 20th century.”
33Precious Bodily Fluids Some people have decided that fluoridation of the water is bad. In the 1950’s US, some thought it was a communist conspiracy. Now some people think it’s a conspiracy of the UN or the “New World Order” or whatever.
34Cherry Picking“The 2011 figures show little difference [in cavity-free 5 year olds], with the rate for fluoridated areas being 59.91% and that for non-fluoridated areas being 59.18%.” (See course website for reading.)
38Confirmation BiasPeople tend to look for evidence that agrees with what they already believe. This is called confirmation bias.
39Bacon on Confirmation Bias “The human understanding when it has once adopted an opinion… draws all things else to support and agree with it. And though there be a greater number and weight of instances to be found on the other side, yet these it either neglects and despises, or else by some distinction sets aside and rejects; in order that by this great and pernicious predetermination the authority of its former conclusions may remain inviolate.” – Sir Francis Bacon
41Confirmation Bias“Confirmation bias is perhaps the best known and most widely accepted notion of inferential error to come out of the literature on human reasoning.” (Evans, 1989, p. 41)
42Confirmation Bias“Many have written about this bias, and it appears to be sufficiently strong and pervasive that one is led to wonder whether the bias, by itself, might account for a significant fraction of the disputes, altercations, and misunderstandings that occur among individuals, groups, and nations.” – Nickerson 1998
43It’s Ordinary“Your day might start on a sour note: you wake up late, the barista screws up your coffee (even after you’ve waited forever in line), your car won’t start again. If you mentally set yourself up to say “This is a bad day,” you will look for further evidence to support that notion throughout your day, potentially ignoring evidence that it’s a day like any other or maybe even a good day overall.” -- Satya Putumbaka
45Fish vs. Obama“There’s no mistaking what’s going on in the speech delivered last week. No preliminary niceties; just a rehearsal of Obama’s actions and expectations. Eight ‘I’’s right off the bat:” -- Stanley Fish
46“I”s“Just over two months ago I spoke with you… “ “and I laid out what needed to be done.” “From the beginning I made it clear that I would not put any more tax dollars on the line.” “I refused to let those companies become permanent wards of the state.” “I refused to kick the can down the road.” “But I also recognized the importance of a viable auto industry.” “I decided then…”
61A Strange ExampleThey said (1), West Germany and East Germany. Others (Americans) were also asked:Which of these pairs of countries are more different from one another?West Germany, East GermanySri Lanka, Nepal
62A Strange ExampleThey also said (1). Americans thought that West Germany and East Germany were both more similar to each other than Sri Lanka and Nepal and less similar to each other than Sri Lanka and Nepal. How is that possible?
63First, when considering the question ‘which are more similar First, when considering the question ‘which are more similar?’ the subjects looked for all the positive evidence that West Germany and East Germany were similar, and all the positive evidence that Sri Lanka and Nepal were similar. Since Americans know nothing about Asian countries, they had no positive reason to think Sri Lanka and Nepal were similar.
64Similarly, when asked ‘which are more different Similarly, when asked ‘which are more different?’ the subjects considered the positive evidence that West Germany and East Germany were different and the positive evidence that Sri Lanka and Nepal were different. Again, having no knowledge of Sri Lanka or Nepal, Americans chose (1), because of all the positive evidence in its favor.
65But it cannot be true that East Germany and West Germany are both more similar and more different than Sri Lanka and Nepal. What the subjects did not do is consider the relevant negative evidence that would disconfirm their hypotheses.
67The Problem of Absent Data Sometimes it’s not just that we only look for or evaluate the positive evidence, but that there is no negative evidence. This can lead us to think we have very well-confirmed beliefs when we do not.
68Hiring Job ApplicantsSuppose you’re hiring job applicants for your shoe company. You think people who haven’t studied a musical instrument would not be good employees. Who do you hire? The people who have studied music, of course! And if they’re successful at your company, do you have good reason to believe that you were right?
69No! You have positive evidence– people you predicted would be successful, who are successful– but you have no negative evidence. What about all the people you didn’t hire, the ones who didn’t study music? They might have been successful too. They could’ve been more successful. You don’t know.
70Absent evidence is all around us Absent evidence is all around us. Suppose you decide to major in accounting instead of philosophy. You find that you are very happy studying accounting. Did you make the right choice? You can’t know. You could have been more happy studying philosophy. There’s just no evidence.
71The Echo ChamberData can be absent also because we make it so: we put ourselves in a position to never hear people who disagree with us.