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Math Help - Help with P-Value

  1. #1
    Junior Member arcticreaver's Avatar
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    Help with P-Value

    So i get a problem where I must find the p-value.

    I am given the following information:

    x=21
    n=25
    standard deviation is 5

    H0: u=20
    H1: u not equal to 20

    so i'm confused. since it does not give me a t value to base this stuff off of like .5 or .025, etc.

    don't i need that to solve this problem?
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  2. #2
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    Re: Help with P-Value

    P-Value is the probability of your x given the Null hypotheses (H0) is true.

    Here your H0 is Normal(20,5/sqrt(25)) (do you understand why?) = Normal(20,1)

    Normalize x to obtain ((x - mean)/stdev), in this case ((21 - 20)/1) = 1

    and use your tables or whatever to find the Probability of observing 1 under the Normal(0,1) distribution.


    I get 0.841
    Last edited by romsek; December 6th 2013 at 11:06 AM.
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  3. #3
    Junior Member arcticreaver's Avatar
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    Re: Help with P-Value

    right 1 is 0.841

    so now i need to subtract 1 - .841, right? with is .159, but this is not the correct p-value, since the answer it's giving me is like .3174...
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    Re: Help with P-Value

    Quote Originally Posted by arcticreaver View Post
    right 1 is 0.841

    so now i need to subtract 1 - .841, right? with is .159, but this is not the correct p-value, since the answer it's giving me is like .3174...
    0.841 is your p-value for your observations x=21
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  5. #5
    Junior Member arcticreaver's Avatar
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    Re: Help with P-Value

    well that is not what the answer key is saying.

    it says the P-Value is .3174
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  6. #6
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    Re: Help with P-Value

    Quote Originally Posted by arcticreaver View Post
    well that is not what the answer key is saying.

    it says the P-Value is .3174
    Ok, I misinterpreted what I was reading. Because of the way this problem is set up as the hypotheses being u=20 and u <> 20

    You need to take the sum of the 2 two tails i.e. 2 (1 - Pr[x])

    Here is an example of the type of problem you are doing with an explanation.
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  7. #7
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    Re: Help with P-Value

    Quote Originally Posted by romsek View Post
    Ok, I misinterpreted what I was reading. Because of the way this problem is set up as the hypotheses being u=20 and u <> 20

    You need to take the sum of the 2 two tails i.e. 2 (1 - Pr[x])

    Here is an example of the type of problem you are doing with an explanation.
    This is a better explanation. Basically because this test is a u=20 vs. u<>20 test you need to use a 2 tailed test.
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  8. #8
    Junior Member arcticreaver's Avatar
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    Re: Help with P-Value

    omg, thanks for that LINK!!!! i get it now!
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