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December 3rd, 2011, 05:50 AM   #1
Joined: Jan 2011

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Confidence intervals [stats]

Hi, I've been doing some questions on confidence intervals (stats 3) and I've come along a question which I believe i have done with the correct method, however after checking the answers (at the back of the book) they seem to have a different answer.

Anyway, the question is as follows:

5 masses are normally distributed with mean ? and standard deviation ?. The 5 randomly chosen masses are (grams)
124.31 125.14 124.23 125.41 125.76

Calculate the symmetric 95% confidence interval for the mean mass of the population

a. assuming that ? = 0.85
b. without assuming that ? = 0.85

Ok so i can do part a easily, however for part b, my method is first finding the variance (using sum of x^2/n - mean^2) and going about finding the confidence interval in the usual manner.

So my answer was [124.43, 125.50] but this isnt what is at the back of the book.

Does anyone know if maybe my method is wrong? (i have find a normal variance, rather than an unbiased estimate because the sample of 5 is not classified as large)
or maybe if anyone could work through and share their solution?

Thanks in advance
jackinda is offline  
December 4th, 2011, 02:39 AM   #2
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Re: Confidence intervals [stats]

It would help a whole heck of a lot if you'd show your work instead of just your answers -- I have no idea where you got those from. I get much closer to your answer using z instead of t, and by using variance instead of standard deviation, though I still can't find your answers. It would also help if you gave the answers the book is giving, just to check to see if the method I would use agrees with your book's answer.

I have no idea what "sum of x^2/n - mean^2" is. Without parenthesis, not sure how to interpret that and I'm not familiar with it. I use

for variance, and the square root of that for standard deviation. And I'm getting .6764 for s.d.

Then I'm using

Then I'm using a t chart since n=5 (and there's no sigma), at 95%, d.f.=4.

And I'm getting .

Are you using these methods?
Erimess is offline  

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