Thread: 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.

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.

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

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?

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?

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.

Originally Posted by TKHunny

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 where is the population standard deviation. If is unknown, it needs to be estimated by . If is large ( , although some books will say ) then 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.

Originally Posted by ajoduren

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.