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Homework Help: Normal/t distributions

  1. Jul 11, 2013 #1
    1. The problem statement, all variables and given/known data

    Let X1, X2,...,X16 be a random sample of size 16 from N(μ=50, σ2=100) distribution.

    Find P(Xbar > 50 + .6505(s))

    2. Relevant equations

    Z= (xbar - μ)/(σ/√n)

    3. The attempt at a solution

    So I know the solution of this problem is given by P( T(15 d.f.) > 2.602 ) but I'm not sure why.

    I notice that you get the value 2.602 by plugging (50 + .6505(s)) in for xbar in the Z equation.

    I guess I'm not really sure why we use the T distribution here and how you're supposed to come up with the value 2.602. 15 degrees of freedom makes sense.
  2. jcsd
  3. Jul 11, 2013 #2


    Staff: Mentor

    You have to use the Student's t distribution because your sample size is so small. As I recall the cutoff is about n = 28. For your problem, n = 16.
  4. Jul 11, 2013 #3
    Ok. That makes sense.

    I suppose I'm confused because in the previous problem we were asked to find P(xbar >52) and we used z scores/normal cdf
  5. Jul 11, 2013 #4

    Ray Vickson

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    What, exactly, do you mean by 's' in the expression Xbar > 50 + 0.6505 s ? If 's' is the sample standard deviation, you need to use the t-distribution because you are essentially asking for the distribution of T = (Xbar - μ)/s (or maybe (Xbar - μ)/(s/√n), depending on what s actually represents). You would use the normal distribution if you wanted Xbar > 50 + 0.6505 σ with known σ = 10. In other words, use N for (Xbar - μ)/σ and use T for (Xbar - μ)/s.
  6. Jul 11, 2013 #5
    's' represents the sample standard deviation.

    I'm still a bit unclear as to how we come up with the value 2.602
  7. Jul 11, 2013 #6

    Ray Vickson

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    [tex] 2.602 = 4 \times 0.6505 = \sqrt{16} \times 0.6505 [/tex]
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