# Tag Archives: probability

## the World Cup and probability: a lost opportunity?

I was disappointed in the amount of press given to silly predictions during the World Cup.  Or rather, I was disappointed that the silly predictions did not lead to a greater discussion of probability and conditional probability.

First, we have Paul the Octopus.  Wikopedia claims that he was correct in all eight of his World Cup predictions, and that he is 12/14 overall.  Getting all eight predictions roughly has a probability of (1/3)^3 * (1/2)^5 = 1/864 (the first three predictions could have resulted in a tie).  Not bad. But Paul the Octopus gained international fame after his first four correct predictions, which means the conditional probability that his last four predictions are correct given that his first four predictions were correct is (1/2)^4 = 1/16.  It was unexpected that he would continue to make correct predictions after attaining such fame, but given that there were certainly more than 864 bizarre ways to make World Cup predictions, someone had to get them all right.  Still, the octopus is a great mascot.

Next, we have Mick Jagger, who was declared a jinx after all three of the teams he supported lost.  Of course, this was done after the fact.  Given the large number of celebrities who attended World Cup games, it is not surprising that a few saw their teams(s) win more than others.  So that’s my way of saying that I’m not seeing why Mick Jagger was newsworthy.

Are there any other examples of probability–good or bad–that hit the mainstream during the World Cup?  Was there any useful discussion of probability during the World Cup?  How did you make World Cup predictions (aside from relying on octopuses)?

## Bulgarian lottery – what are the odds?

The same six winning lottery numbers turned up in two consecutive drawings in the Bulgaria lottery earlier in the month (1 chance in 5.2 million).  Carl Bialik in the WSJ writes about the odds of this happening.  He notes that “With so many numbers colliding each week, the lottery might be the ideal proving ground for something that statisticians have long recognized: Given enough opportunities, the seemingly impossible becomes plausible.”  He explores several lottery issues in more detail in the Numbers Guy blog.  Statistician David Smith also blogged about the Bulgarian lottery.

Although the lottery is random, the people who play it are not.  I had always intuitively known this, but the picture below illustrates this quite nicely.  Apparently, people making lottery picks based on birthdays, for example, skews the picks toward smaller numbers.

Lottery numbers as chosen by lottery players are far from random

The lotteries are designed such that the expected winnings are negative when accounting for the price of the ticket, since the probability of winning is so low (E[winnings] = P(win)*Jackpot – Ticket Price). When the jackpot grows large enough, the “average” lottery player can come out ahead (although there really is no one at the average – there are a couple of winners who really skew the average). In March 1992, the Virginia lottery almost guaranteed a true winner. It offered a jackpot of \$27M to a single winner whereas it cost \$7.5M to purchase all Choose(44, 6) combinations of possible tickets (by piacking six of 44 numbers). Of course, this strategy could backfire if there were many winners. However, a group of 2500 people accepted this challenge and pooled their resources. They ended up being the single winner, and after a legal struggle, they were awarded the jackpot. The Virginia lottery was subsequently changed to be less lucrative.

Do you play the lottery?