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Statistics, hypothesis test

  1. Feb 5, 2010 #1
    1. The problem statement, all variables and given/known data

    The electricity usage (KWh) in homes was studied over a period of time.

    For Year 1:
    Mean = 1130
    Std dev. = 196
    n = 16

    For Year 2:
    Mean = 1350
    Std dev. = 302
    n = 21

    a)
    Test the hypothesis that the std dev. in year 2 has increased compared to year 1. Remember to state your significance level.

    b)
    Test the hypothesis that the std dev. in year 1 and 2 are equal. Remember to state your significance level.

    2. Relevant equations

    3. The attempt at a solution

    I don't know about a) really.

    For b) I think the F-distribution could be used, right?

    But what significance level should I use? Someone help me get started on this?
     
  2. jcsd
  3. Feb 5, 2010 #2

    Mark44

    Staff: Mentor

    I think that the F statistic is the one to use for both tests. My only concern is the relatively small sample sizes for each problem part.

    From what you have posted, it looks like it's your choice on the significance level. Pick one, but be sure to state it in your work.

    For the first part, the null and alternate hypotheses are these:
    H0: s12 <= s22
    Ha: s12 > s22

    Because of the > in the alternate hypothesis, you need to use a one-tailed test.

    For the second part, the null and alternate hypotheses are these:
    H0: s12 != s22 (i.e., they're not equal)
    Ha: s12 = s22
    For this problem you need to use a two-tailed test.
     
  4. Feb 5, 2010 #3

    statdad

    User Avatar
    Homework Helper

    The F distribution can be used, but the hypotheses in Mark44's post should involve [tex] \sigma_1^2[/tex] and [tex] \sigma_2^2 [/tex], and not the sample variances. I think he just made a quick typo.
     
  5. Feb 6, 2010 #4
    All right, thanks!
    But do you think I should use alfa = 5%, or is another number better?
     
  6. Feb 6, 2010 #5

    Mark44

    Staff: Mentor

    As far as I can tell, it's your call on what to choose for alpha.
     
  7. Feb 7, 2010 #6
    Just one last question
    Since s(y) > s(x)
    where x= year 1 and y= year 2

    In a)
    should I reject H(0) if

    s(x)^2 / s(y)^2 < F n(x)-1, n(y)-1, (a)
    or reject if
    s(x)^2 / s(y)^2 > F n(x)-1, n(y)-1, (a)?
     
  8. Feb 7, 2010 #7

    Mark44

    Staff: Mentor

    Draw the curve for each situation, and determine what the critical values are. If your statistic lies in the tail (one-tailed test) or in either tail (two-tailed test), reject the null hypothesis. Otherwise, fail to reject the null hypothesis.
     
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