628: Psychic

Explain xkcd: It's 'cause you're dumb.
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You can do a lot better than 1% if you start keeping track of the patterns in what numbers people pick.
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Title text: You can do a lot better than 1% if you start keeping track of the patterns in what numbers people pick.


A psychic is a person who claims to be able to access information that is beyond normal sensory perception through extrasensory perception. This information may vary widely in scope and value, ranging from archaeological to the ability to read minds. Cueball describes himself as such a person, to which Megan responds with disbelief because it is a bold and unsupported claim. To prove his abilities, Cueball has Megan think of a random number from 1 to 100, which he then guesses correctly to demonstrate his ability to read minds. Megan is amazed that Cueball was correct, but he simply dismisses her disbelief and wants to go back to pretending to lead a normal life.

The four panels are actually a setup to the real joke in this comic: the final sentence spoken by a narrator. It reveals that Cueball has simply played a trick on Megan and that anyone can repeat it. The joke is that, theoretically, a person can guess a random number from 1 to 100 once in one hundred tries, or 1% of the time, according to the law of large numbers. By playing this trick enough times on enough friends, the trickster is statistically likely to get a number right eventually. Assuming the person whose number he guesses is not familiar with the trick, it will appear as if the trickster is actually psychic. Should this happen, the trickster can then play the joke out as he wants, hence the "it's totally worth it" at the end.

The title-text appeals again to statistics. If the trickster first observes his friends' number guessing patterns without giving away his intentions, his guessing will not be completely random and will be slightly likelier to land on the correct value. Randall suggests that if the trickster were dedicated enough, he could observe his friends long enough to improve the probability of being right to well above 1%.


Man: I'm psychic, you know.
Woman: There's no such thing.
Man: Okay, think of a number from one to one hundred.
Woman: Okay.
Man: 43.
Woman: Holy shit!
Man: I try not to let it affect my life too much.
Woman: Wait, I can't believe this.
Man: Don't worry about it. Forget I said anything.
Woman: But--
Man: Let's get to the movie.
Woman: I, uh... Ok, sure.
Narrator: This trick may only work 1% of the time, but when it does, it's totally worth it.

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Tip: No one picks 50. 01:35, 14 March 2013 (UTC)

The Answer to the Ultimate Question of Life, the Universe, and Everything is 42. This comic is wrong ;) --Dgbrt (talk) 18:52, 6 October 2013 (UTC)

Interestingly, it appears Megan was trying to trip Cueball up. The obvious choice would have been "42", a number with very geeky connotations, to say the least. Megan may have thought of it immediately, known Cueball would suspect, and gone for the next higher number, 43. Of course, Cueball was smart enough to realize this simple trick, and knew Megan was, too. So he won anyway. (talk) (please sign your comments with ~~~~)

I seems that 37 (or 73) would be most not-random random. Though I cannot say, that sources are 100% reliable: [1] [2] 08:57, 15 May 2015 (UTC) Koovert

Wait, if Cueball's probability to pick a specific number is 1/100 and Megan's probability to pick a specific number is also 1/100, wouldn't the probability of their picks being the same be 0.01%? --Jogerj (talk) 02:11, 12 August 2015 (UTC)

Yes, now multiply it by 100 because there can be 100 specific numbers. Anachor (talk) 10:17, 27 August 2015 (UTC)
No, because you only have to have one of them match the other, not both of them match a number from some other source. 07:07, 30 August 2015 (UTC)

Ooh, so the 17 thing is real after all! I've long since spotted it, but was never sure whether it's objective or just me preferentially noticing this number. Man, it feels good to be vindicated. 05:11, 26 April 2016 (UTC)