Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

What is the probablity of generating the same number(s) from random draw, over again?

  1. Sep 8, 2012 #1
    This is a question relating to lotteries. Bascially everyone says that each draw is totally independant of each other. So the probability of generating a sequence of numbers each time is the same no matter what the numbers are. However for two draws, there must be a way to calculate the probability that the same set of numbers would be drawn a second, third, fourth... time. This event would be extremely unlikely yet still could occur.
    Is ther a value we can give to these unlikely occurences?
     
  2. jcsd
  3. Sep 8, 2012 #2

    mathman

    User Avatar
    Science Advisor
    Gold Member

    Re: What is the probablity of generating the same number(s) from random draw, over ag

    Let P be the probability that a set is drawn. Then the probability that, if a set is drawn, it is repeated on the next draw is P. On the next two draws it is P2, etc.
     
  4. Sep 8, 2012 #3
    Re: What is the probablity of generating the same number(s) from random draw, over ag

    Thanks
    So then why is it each draw is considered independant of each other, if say the next draw probability P is dependant on not having the exact same numbers as the previous draw (since otherwise it would be P2
     
  5. Sep 9, 2012 #4

    mathman

    User Avatar
    Science Advisor
    Gold Member

    Re: What is the probablity of generating the same number(s) from random draw, over ag

    Please clarify your question.
     
  6. Sep 9, 2012 #5
    Re: What is the probablity of generating the same number(s) from random draw, over ag

    I guess I'm referring to dependant vs independant events. If it is claimed that lottery draws are independant of each other (i.e. one draw has no influence on the outcome of the next draw) However doesnt this contradict with the change in probability for two consecutive draws where the event is identical?

    Say I play the same lottery numbers every week. The probability that they draw my numbers is P. I continue to play my numbers the following week but the probablity that they draw my numbers on the next draw doesn't remain P, doesn't it change to P2 as was stated in the previous response? If so wouldn't that mean there is some dependance on the previous draw (because the probability changed)?
     
  7. Sep 9, 2012 #6
    Re: What is the probablity of generating the same number(s) from random draw, over ag

    I don't see where he said that anywhere. He said that the probability of a specific sequence being repeated on the next two trials is P2, which is true.
     
  8. Sep 9, 2012 #7
    Re: What is the probablity of generating the same number(s) from random draw, over ag

    The odds of drawing number x is p. The odds of drawing it twice is P squared but only if this is specified in advance.

    If you draw x then the probability of drawing x the second time is p.
     
  9. Sep 9, 2012 #8
    Re: What is the probablity of generating the same number(s) from random draw, over ag

    Is this correct?

    1st attempt => draw numbers N = probability P
    2nd attempt => draw same numbers N = probability P
    3rd attempt => draw same numbers N = probabiloty P2
    4th attempt => draw same numbers N = probability ?

    What does this mean, specified in advance?
     
  10. Sep 9, 2012 #9
    Re: What is the probablity of generating the same number(s) from random draw, over ag

    No. The probability is P at every step.
     
  11. Sep 10, 2012 #10
    Re: What is the probablity of generating the same number(s) from random draw, over ag

    So then what was P2?
    I'm really confused now.
     
  12. Sep 10, 2012 #11
    Re: What is the probablity of generating the same number(s) from random draw, over ag

    anytime you draw, the probability of getting x is P, but the probability of getting x twice is P2 This -requires- looking at two draws, not just one. P is for one draw, any one draw, but not TWO draws. methinks. Perhaps it is three...I would have thought two. Clarification there? But certainly more than one! And that is what changes the probability, the number of draws you are looking at
     
  13. Sep 10, 2012 #12
    Re: What is the probablity of generating the same number(s) from random draw, over ag

    Let's use coins. The probability of flipping tails is 0.5 every time regardless of the history of the prior flips.

    But if you specify in advance that you wish to flip 10 tails in a row, the probability of that entire sequence of flips is 1/2^10, about 1024:1.

    Once you have reached the ninth toss and it has come up tails, then the probability of flipping the tenth tail is 0.5 even though your odds of getting to the ninth flip were 512:1.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: What is the probablity of generating the same number(s) from random draw, over again?
  1. Random Number generator? (Replies: 12)

Loading...