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Math Help - If the E-value of a match is 1E-23, what is the probability...

  1. #1
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    If the E-value of a match is 1E-23, what is the probability...

    If the E-value of a match is 1E-23, what is the probability of 1 or more matches this good or
    better for a random query sequence?

    So I did 1- e^(-1E-23)= the answer
    I got zero for an answer and my teacher says "in spite of what your calculator says, the answer to is not zero."

    Help?

    Link to the worksheet: 2011-02-15_2205 - trinhn812's library
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  2. #2
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    hmm actually

    Actually I think the answer is 1E-23 since the answer is so small the calculator rounds it or something
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  3. #3
    Grand Panjandrum
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    Quote Originally Posted by okgo123 View Post
    If the E-value of a match is 1E-23, what is the probability of 1 or more matches this good or
    better for a random query sequence?

    So I did 1- e^(-1E-23)= the answer
    I got zero for an answer and my teacher says "in spite of what your calculator says, the answer to is not zero."

    Help?

    Link to the worksheet: 2011-02-15_2205 - trinhn812's library
    Put your calculator away.

    We are dealing with a Binomial distribution with an unknown number of Bernouli trials and single trial success probability so:

    If the probability of a match with a single random record is $$p and there are $$n records then:

    E=np=1\times 10^{-23}

    Now the probability on no matches in $$n records is (since $$np is small):

    pr(k=0;n,p)=1-pr(k\ge1;n,p) = 1 - (1-p)^n \approx 1 - (1-np)=np= 1\times 10^{-23}

    CB
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