Thursday, June 18, 2009

How to pull it off

At the NY Area Theory Day, I started my talk with a story. When I was 10 years old, I played this game where the other party will repeatedly toss a coin, and I would predict the outcome each time. I developed a trick to predict correctly significantly more than 50% of the times. I told the trick to the audience and lost many of them who spent the rest of the talk wondering or disagreeing that it would work. Graham Cormode pointed out that besides the trick, the challenge is to pull it off since it required some worldly social skills.

I was reminded of this when I read about a scam in India. Roughly, via email solicitation, for a fee, someone delivers a trunk full of blackened currency to you. And then asks you for money to send you chemicals that will clean and resurrect the bills. Of course the bills turn out to be bogus. This scam seemed silly. I would have liked to see one in which you get delivered the trunk full of blackened currency free of charge, then you are asked to pay a small amount and get delivered a small quantity of chemicals that cleans and retrieves a few genuine or high quality bogus currency bill, and finally the victim gets hit for a larger batch, producing only counterfeit or bogus notes. No, I am not recommending such a scheme, just pointing out that the scheme could be made more nuanced.


Blogger rrenaud said...

I am curious about your trick.

Is there some correlation between flips?

Did you get to control the stopping point? That is, you flip a coin an undetermined number of times, and then only stop flipping once you have predicted them correctly more than half the time?

8:27 PM  
Anonymous Anonymous said...

I will send you the trick in a little while, dont want to spoil the fun. :)

-- metoo

8:33 PM  
Blogger Panos Ipeirotis said...

Come on, give us the trick.

We promise that we will be reading the next blog posts and we will not be arguing in the comments...

2:31 AM  
Anonymous Anonymous said...

Since this is a serious game (not just kids tossing coins in a playground!), you lay down the rule first that the tosser should toss the coin in a systematic way, say to a reasonably appropriate height each time. Then, you make sure you observe the "top" side of the coin before the toss begins each time.

Now the rest is easy. Almost everyone has a bias when they toss a coin systematically: the coin falls the top side up more than 50% of the times, or the top side down more than 50% of the times, depending on the tosser. You can figure out the bias over time, and call it with some noise that you throw in.

-- metoo

ps: Panos, remember your promise!

6:30 AM  

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