Thursday, March 1, 2012

Suppose you flip a fair coin (probability of heads = probability of tails) 6 times and the coin lands "tails" face up each time.  What is the probability that the next toss will also be a tail? What is the probability that the next toss is a head?

One might think that it's more likely to get a head since the first 6 tosses were tails, but in fact the probability that the next toss is a tail is .5, as is the probability that the next toss is a head.  The coin has no memory, so to speak. Even if the first 100 tosses landed tails up, the next toss still has a 50 percent chance of landing tails up.

Now the probability that all 6 coins tossed landing tails up is (1/2)(1/2)(1/2)(1/2)(1/2)(1/2) = 1/64.

Each toss of the coin is an independent event. What occurred previously on the coin toss has no bearing on the outcome of the next coin toss.

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