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Thread: Z-values and probability

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    Z-values and probability




    Suppose concentration of arsenic (in ppb) in groundwater sampled quarterly from a particular well can be modeled with a normal distribution with a mean of 5 and a standard deviation of 1.

    a) What is the probability that a single observation will be given greater than 7ppb?

    z =5-7 / 1 = 2 And since we want greater than the probability would be 2.28.

    http://imgur.com/r8jzeo4

    What I'm confused on is the second part!
    b)What is the probability that the average of four observations will be greater than 7 ppb?

    So now, I have to take into account n, so the formula shifts from
    z =Xi-7/ sd to z =xbar-7 / [sd/sqrt(n)]

    z =5-7 /[1/sqrt(4)]= -4 Therefore, the probability is 100?
    Which doesn't make sense at all.
    Last edited by chickennugget; 03-09-2014 at 09:35 PM.

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