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Poisson Distribution

  1. May 5, 2009 #1
    1. The problem statement, all variables and given/known data

    A source of liquid is known to contain bacteria, with the mean number of bacteria per cubic centimeter equal to 3. Ten 1 c.c. test tubes are filled with liquid. Calculate the probability that all 10 test tubes will show growth, that is contain at least 1 bacterium each. (use Poisson distribution)

    2. Relevant equations
    P(r)=(e-m*mr)/r!

    3. The attempt at a solution
    Taking m=3;
    N=10 (not used)
    P(1)+P(2)+P(3)=0.5974

    Answer provided is 0.600, is my solution correct?
     
  2. jcsd
  3. May 5, 2009 #2

    Mark44

    Staff: Mentor

    No. The probability of at least 1 bacterium in a test tube is P(1) + P(2) + P(3) + P(4) + P(5) + ... Why did you stop at 3 bacteria?

    The probability I showed is equal to 1 - P(0), which is about .9502. This is the probability of finding 1 or more bacteria in one test tube. Now, how do you get the probability of finding 1 or more bacteria in all 10 test tubes?

    The answer I got was .6001, rounded to 4 decimal places.
     
  4. May 5, 2009 #3
    Taking p=0.9502 as a success, n=10 for binomial distribution:
    P=10C10*p10*q0
    P=0.600.

    Is this correct?
     
  5. May 5, 2009 #4

    Dick

    User Avatar
    Science Advisor
    Homework Helper

    Yes.
     
  6. May 5, 2009 #5
    Thanks.
     
  7. Jun 15, 2009 #6
    I'm working on a similar problem. Can someone tell me where this equation came from? What's c and q (and what do those 2 10's next to the c mean)?
     
  8. Jun 15, 2009 #7

    Mark44

    Staff: Mentor

    C stands for combinations. The first expression in this binomial probability is 10C10, which is the number of combinations of 10 things taken 10 at a time, which turns out to be 1. p is a probability of something happening (you didn't give any context here, so I can't say anything more) and q is the probability of something not happening, which means that q = 1 - p.
     
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