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Homework Help: A Couple of Probability & Statistics Questions

  1. Jan 13, 2009 #1
    1. The problem statement, all variables and given/known data
    I have a table with a data set #, and 2 variables (W and P) with values in the table.

    2. Relevant equations
    The equation (which I don't know what it means) says V= (summation with n on top and i=1 below Wi X Pi)/(summation with n on top and i=1 on bottom X Pi)

    3. The attempt at a solution

    What are the summations meaning and what do the subscript "i"'s mean?
  2. jcsd
  3. Jan 13, 2009 #2
    Questions 2 is "You are reviewing a business decision where you have calculated the profit for 5 possible business scenarios. If the probabilities of occurrence for four of the scenarios are 12%, 10%, 21%, and 30% respectively; what is the percent probability of occurence for the fifth scenario.

    Does this question even make sense?
  4. Jan 13, 2009 #3
    I read that as "there are exactly 5 possible scenarios". In a valid probability-model, what do you know about the sum of all possible probabilities?

  5. Jan 13, 2009 #4
    27%. I love when they are easier than you think!! Thank you!

    Anybody have an idea about the first one?
  6. Jan 13, 2009 #5


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    Gold Member

    What is X? Should that just be Pi?

    Anyway, the top summation means that you are starting from the "i=1" data point (i.e. the first W and P), multiplying W and P, then doing the same for the second data points (i=2) all the way up to the n'th W and P, and then adding them all up.

    In general,

    [tex]\sum _{i=a} ^{n} f(i) = f(a) + f(a+1) + f(a+2) + ... + f(n) [/tex]

    So it just means that you start with i=a, and plug it into the function f(i). Then continuously increase 'i' by integer steps until you get to i=n, each time plugging it into the function f(i). Then simply add each f(i) that you calculated along the way.

    So in your cases:

    [tex]\sum _{i=1} ^{n} W_i P_i = W_1P_1+W_2P_2+...+W_nP_n [/tex]

    [tex]\sum _{i=1} ^{n} P_i = P_1+P_2+...+P_n [/tex]
    Last edited: Jan 13, 2009
  7. Jan 13, 2009 #6
    Awesome! Thank you!
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