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Math Help - [SOLVED] 2-sided confidence interval for small sample

  1. #1
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    Red face [SOLVED] 2-sided confidence interval for small sample

    33. degrees of polymerization are:
    418 421 421 422 425 427 431

    434 437 439 446 447 448 453

    454 463 465

    c. Calculate a two-sided 95% confidence interval for true average degree of polymerization.


    n=17 (so I will calculate a t-confidence interval)
    t-confidence interval= mean +/- (t-critical value * s/√n)

    I have calculated ∑x=7,451 so ∑x=55,517,401. So the mean is 7,451/17≈ 438.29. t-critical value is 2.11. √n=√17≈4.123.
    If I understand this, then I just need to calculate s to attain the confidence interval. However, I don't know how to calculate s without having the probability of each x. (i.e. √σ=E(x)-[E(x)]).

    Please let me know how my logic is failing and what I need to do to solve this problem. I have spend several hours on this.

    Thanks a bunch!
    -Yvonne
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  2. #2
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    Smile formula for sample standard deviation

    I found my answer:

    s=√[(∑(x-mean))/n-1)]
    s=15.14


    Hope this helps others.
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