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Math Help - 1 in 100 Chance?

  1. #1
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    1 in 100 Chance?

    I was browsing the forums of World of Warcraft, and it was mentioned in a thread there that the drop rate of something was 1 in 100. Someone was asking, why is it that, after 100 times going for this item 100 times, I still don't have it? Well, i thought the answer was obvious, that as it's on an Random Number Generator then its a different roll each time, and theres just a higher chance it will roll what you want and the item will drop. However, this other person said,

    Quote: "You don't know probability. If you run something 100 times with a 1 in 100 chance, that only equates to a ~63% chance it will drop.".

    Can someone please explain to me how this works?
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  2. #2
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    Quote Originally Posted by mcaelli View Post
    I was browsing the forums of World of Warcraft, and it was mentioned in a thread there that the drop rate of something was 1 in 100. Someone was asking, why is it that, after 100 times going for this item 100 times, I still don't have it? Well, i thought the answer was obvious, that as it's on an Random Number Generator then its a different roll each time, and theres just a higher chance it will roll what you want and the item will drop. However, this other person said,

    Quote: "You don't know probability. If you run something 100 times with a 1 in 100 chance, that only equates to a ~63% chance it will drop.".

    Can someone please explain to me how this works?
    The probability of 1 or more success in 100 trials with probability of success 0.01 is:

    p=1-b(0,100,0.01)=1-(0.99)^100\approx 0.634

    where b(n,N,p) denotes the binomial probability of n successes in N trials with probability of success on a single trial of p.

    Another way of thinking of this is the probability of one or more success in 100 trials is 1 minus the probability of 100 failures.

    CB
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