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Positive Bias

Just wanted to post this one here, so that I remember to try it with someone next time!!


"This is a game based on a famous experiment called the 2-4-6 task, and this is how it works. I have a rule - known to me, but not to you - which fits some triplets of three numbers, but not others. 2-4-6 is one example of a triplet which fits the rule. In fact... let me write down the rule, just so you know it's a fixed rule, and fold it up and give it to you. Please don't look, since I infer from earlier that you can read upside-down."

The boy said "paper" and "mechanical pencil" to his pouch, and she shut her eyes tightly while he wrote.

"There," said the boy, and he was holding a tightly folded piece of paper. "Put this in your pocket," and she did.

"Now the way this game works," said the boy, "is that you give me a triplet of three numbers, and I'll tell you 'Yes' if the three numbers are an instance of the rule, and 'No' if they're not. I am Nature, the rule is one of my laws, and you are investigating me. You already know that 2-4-6 gets a 'Yes'. When you've performed all the further experimental tests you want - asked me as many triplets as you feel necessary - you stop and guess the rule, and then you can unfold the sheet of paper and see how you did. Do you understand the game?"

"Of course I do," said Hermione.

"Go."

"4-6-8" said Hermione.

"Yes," said the boy.

"10-12-14", said Hermione.

"Yes," said the boy.

Hermione tried to cast her mind a little further afield, since it seemed like she'd already done all the testing she needed, and yet it couldn't be that easy, could it?

"1-3-5."

"Yes."

"Minus 3, minus 1, plus 1."

"Yes."

Hermione couldn't think of anything else to do. "The rule is that the numbers have to increase by two each time."

"Now suppose I tell you," said the boy, "that this test is harder than it looks, and that only 20% of grownups get it right."

Hermione frowned. What had she missed? Then, suddenly, she thought of a test she still needed to do.

"2-5-8!" she said triumphantly.

"Yes."

"10-20-30!"

"Yes."

"The real answer is that the numbers have to go up by the same amount each time. It doesn't have to be 2."

"Very well," said the boy, "take the paper out and see how you did."

Hermione took the paper out of her pocket and unfolded it.

Three real numbers in increasing order, lowest to highest.

Hermione's jaw dropped. She had the distinct feeling of something terribly unfair having been done to her, that the boy was a dirty rotten cheating liar, but when she cast her mind back she couldn't think of any wrong responses that he'd given.

"What you've just discovered is called 'positive bias'," said the boy. "You had a rule in your mind, and you kept on thinking of triplets that should make the rule say 'Yes'. But you didn't try to test any triplets that should make the rule say 'No'. In fact you didn't get a single 'No', so 'any three numbers' could have just as easily been the rule. It's sort of like how people imagine experiments that could confirm their hypotheses instead of trying to imagine experiments that could falsify them - that's not quite exactly the same mistake but it's close. You have to learn to look on the negative side of things, stare into the darkness. When this experiment is performed, only 20% of grownups get the answer right. And many of the others invent fantastically complicated hypotheses and put great confidence in their wrong answers since they've done so many experiments and everything came out like they expected."

Harry Potter and the Methods of Rationality, Chapter 8: Positive Bias


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