Date: 03/14/2003 at 14:13:18 From: Kevin Subject: Lottery: Betting same number vs randomly selecting a number. A current co-worker and I are in a friendly disagreement about the probability of selecting the winning number in any lottery, say pick 5. He states that he would rather bet the same set of five numbers every time for x period of time, but I insist that the probability is the same if you randomly select any set five numbers for the same period of time. The only assumption we make here is betting one set of numbers on any given day. Who is correct? I tried explaining to him that the probability of betting on day one is the same for both of us. On day two it is the same. On day three it is the same, etc. Therefore the sum of the cumulative probabilities will be the same for both of us. Thank you for your anticipated response.
Date: 03/15/2003 at 03:29:05 From: Doctor Wallace Subject: Re: Lottery: Betting same number vs randomly selecting a number. Hello Kevin, You are correct. If you have the computer randomly select a different set of 5 numbers to bet on every day, and your friend selects the same set of numbers to bet on every day, then you both have exactly the same probability of winning. Tell your friend to think of the lottery as drawing with tickets instead of balls. If the lottery had a choice of, say, 49 numbers, then imagine a very large hat containing 1 ticket for every possible combination of 5 numbers. 1, 2, 3, 4, 5; 1, 2, 3, 4, 6; etc. On the drawing day, ONE ticket is pulled from the hat. It is equally likely to be any of the C(49,5) tickets in the hat. (There would be 1,906,884 tickets in the hat in this case.) Since both you and your friend have only ONE ticket in the hat, you both have the same chance of winning. On the next drawing day for the lottery, ALL the tickets are replaced. Each lottery draw is an event independent of the others. That is to say, the probability of any combination winning today has absolutely NO effect on the probability of that or any other combination winning tomorrow. Each and every draw is totally independent of the others. The reason your friend believes that he has a better chance of winning with the same set of numbers is probably due to something called the "gambler's fallacy." This idea is that the longer the lottery goes without your friend's "special" set of numbers coming up, the more likely it is to come up in the future. The same fallacy is believed by a lot of people about slot machines in gambling casinos. They hunt for which slot hasn't paid in a while, thinking that that slot is more likely to pay out. But, as the name says, this is a fallacy; pure nonsense. A pull of the slot machine's handle, like the lottery draw, is completely independent of previous pulls. The slot machine has no memory of what has come before, and neither has the lottery. You might play a slot machine for 2 weeks without hitting the big jackpot, and someone else can walk in and hit it in the first 5 minutes of play. People wrongly attribute that to "it was ready to pay out." In reality, it's just luck. That's why they call it gambling. :) This used to be a "trick" question on old math tests: "You flip a fair coin 20 times in a row and it comes up heads every single time. You flip the coin one more time. What is the probability of tails on this last flip?" Most people will respond that the chance of tails is now very high. (Ask your friend and see what he says.) However, the true answer is that the probability is 1/2. It's 1/2 on EVERY flip, no matter what results came before. Like the slot machine and the lottery, the coin has no memory. Thanks for writing to Dr. Math. Don't hesitate to write again if you need further help with this or another question. - Doctor Wallace, The Math Forum http://mathforum.org/dr.math/
Date: 03/17/2003 at 09:50:05 From: Kevin Subject: Thank you (Lottery: Betting same number vs randomly selecting a number.) Thank you for your prompt and thorough response. You have helped us rest this case. Keep up the good work. Kevin
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