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

• Feb 15th 2011, 07:12 PM
okgo123
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
• Feb 15th 2011, 09:08 PM
okgo123
hmm actually
Actually I think the answer is 1E-23 since the answer is so small the calculator rounds it or something(Thinking)
• Feb 15th 2011, 10:16 PM
CaptainBlack
Quote:

Originally Posted by okgo123
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

If the probability of a match with a single random record is $\displaystyle $$p and there are \displaystyle$$n$ records then:
$\displaystyle E=np=1\times 10^{-23}$
Now the probability on no matches in $\displaystyle $$n records is (since \displaystyle$$np$ is small):
$\displaystyle pr(k=0;n,p)=1-pr(k\ge1;n,p) = 1 - (1-p)^n \approx 1 - (1-np)=np= 1\times 10^{-23}$