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Math Help - Urgent Homework Help: Confidence Interval

  1. #1
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    Urgent Homework Help: Confidence Interval

    Can someone help me figure out this problem Please?

    A random sample of 10 miuniature Toootsie Rolls wa taken from a bag. Each piece was weighed on a very accurate scale. The results in grams were

    3.087 3.131 3.241 3.241 3.270 3.553 3.440 3.411 3.437 3.477

    (a) contruct a 90 percent confidence interval for the ture mean weight. (b) what sample size would be necessary to estimate the true weight of +_ 0.03 grams with 90 percent confidence? (c) Discuss the factors which might cause a variation in the weight of Tootsie Rolls during manufacture.
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  2. #2
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    You should not be struggling with this. Go ahead and construct the confidence interval.

    Find the Sample Mean.
    Find the Sample Standard Deviation.
    Find the Standard Deviation of the Sample Mean. <== This is the important part that will allow you to answer the second question.
    Decide if you can use a z-score or a t-score.
    Find the right value for a 90% confidence interval. Is it one-tail or two-tail?

    That's a good start. Show us what you get and we can move on from there.
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  3. #3
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    Urgent Homeowrk help

    I added all of the number together and divded by 10 which that equals to 3.0047 is this correct for the sample mean?
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    RE: Urgent Homework help

    Here is what I have so far:

    The sample mean is 3.3288
    The standard deviation is 0.15618
    the upper is 3.4100
    lower 3.2476

    what do I do next?
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  5. #5
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    The first thing you should do is learn how to show your work. How did you get those values?

    Did you use the sample standard deviation or the standard deviation of the sample mean?
    Last edited by TKHunny; February 15th 2008 at 07:04 AM.
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  6. #6
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    Urgent Homework Help Reply

    I think I used the standard deviation mean. I am so confused. If I could look at a similiar problem I think that I can figure it out. I used the confidence inteval using excel.
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  7. #7
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    Quote Originally Posted by TKHunny View Post
    You should not be struggling with this. Go ahead and construct the confidence interval.

    Find the Sample Mean. *
    Find the Sample Standard Deviation. *

    Find the Standard Deviation of the Sample Mean. <== This is the important part that will allow you to answer the second question. ? Mr F says: This is equal to \, \frac{\sigma}{\sqrt{n}} \, where \, \sigma \, is the population standard deviation. If \, \sigma \, is unknown, it needs to be estimated by \, s \,. If \, n \, is large ( \, n \geq 50 \, , although some books will say \, n \geq 30 \, ) then \, s \approx \sigma and you can use the z-test. Otherwise you must use the t-test (assuming a roughly normal distribution).

    Decide if you can use a z-score or a t-score. ?
    Find the right value for a 90% confidence interval. Is it one-tail or two-tail? ?

    That's a good start. Show us what you get and we can move on from there.
    Quote Originally Posted by ajoduren View Post
    I think I used the standard deviation mean. I am so confused. If I could look at a similiar problem I think that I can figure it out. I used the confidence inteval using excel.
    I have put a * next to the 'boxes' I think you've ticked in TKhunny's list and a ? next to the ones that might have been ticked ..... You need to be much clearer in what you've done and why you've done it.
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