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Homework Help: Probability Question

  1. Oct 24, 2011 #1
    1. The problem statement, all variables and given/known data
    In a population of children 60% are vaccinated against the 'waterpokken'. The probablilities of contracting 'waterpokking' are 1/1000 if the child is vaccinated and 1/100 if not.

    a: Find the probability that a child selected at random will contract 'waterpokken'.

    2. Relevant equations
    Bayes Theorem: P(A|B) = P(A/\B) / P(B)
    Formula sheet? View attachment stat_formulas.pdf

    3. The attempt at a solution
    Known values: P(V) = 0.6 | P(⌐V) = 0.4 | P(W|V) = 0.001 | P(W|⌐V) = 0.01

    a: Find the probability that a child selected at random will contract 'waterpokken'.
    1. The question is: what is P(W)?
      From Bayes Theorem I conclude: P(W) = P(V/\W) / P(V|W)
      Both P(V/\W) & P(V|W) are unkown at this stage.

    2. P(V/\W)?
      Fill in known values in bayes theorem:
      P(W|V) = P(W/\V) / P(V) = 0.001 = P(W/\V) / 0.6
      So: P(V/\W) = P(W/\V) = P(V) * P(W|V) = 0.6 * 0.001 = 0.0006
      Until here I get it! But now...

    3. P(V|W)?
      P(V|W) = P(V/\W) / P(W) but I don't know P(W) :S?

      Answer sheet solution:
      P(W/\⌐V) = P(W|⌐V) = 0.01 * 0.4 = 0.004
      P(W) = P(W/\V) + P(W/\⌐V) = 0.0006 + 0.004 = 0.0046

    1. What rules are applied in the third section? Could someone explain to me how they conclude to
    P(W/\⌐V) = P(W|⌐V) and P(W) = P(W/\V) + P(W/\⌐V) ?

    Maybe it's part of the formulas at the bottom of the first page of my formula sheet View attachment stat_formulas.pdf

    2. Is there a way I could have known that I would need to work to that solution. Because I concluded I needed to work towards P(W) = P(V/\W) / P(V|W) but that plainly doesn't work..:(
     
    Last edited: Oct 24, 2011
  2. jcsd
  3. Oct 24, 2011 #2
    is my question not clear enough?:( The exam is tomorrow (tuesday).
     
  4. Oct 24, 2011 #3

    LCKurtz

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    It doesn't require Bayes theorem. Just plug the numbers into

    [tex]P(W) = P(W|V)P(V) + P(W|\overline V)P(\overline V)[/tex]
     
  5. Oct 24, 2011 #4

    HallsofIvy

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    I always prefer to work with integers! Suppose there are 10000 children. 6000 of them are vaccinated, 4000 are not. Of the 6000 who are vaccinated 6 of them contract the disease, 5994. Of the 4000 who are not vaccinated, 40 contract the disease.
     
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