# statistics confidence intervals

• Apr 12th 2011, 06:36 PM
JGaraffa
statistics confidence intervals
so so confused. had to miss a week of class. any guidance is greatly appreciated.

a random sample of 121 checing accounts at a bank showed an average daily balance of $280. the standard deviation of the population is known to be$66

a) is it necessary to know anything about the shape of the distribution of the account balances in order to make an interval estimate of the mean of all the account balances? explain
b) find the standard error of the mean
c) give a point estimate of the population mean
d) determine the margin of error for a 95% confidence interval
e) construct a 95% confidence interval for the mean
• Apr 12th 2011, 07:30 PM
pickslides
Hi there JGaraffa,

Have you thought about this problem yet? Have you attempted these? Have you any class note you can refer to?
Quote:

Originally Posted by JGaraffa

a) is it necessary to know anything about the shape of the distribution of the account balances in order to make an interval estimate of the mean of all the account balances? explain

What data distributions do you know? What does this mean here?

Quote:

Originally Posted by JGaraffa

b) find the standard error of the mean

This is an pretty easy one, $\displaystyle \displaystyle SE = \frac{\sigma}{\sqrt{n}}$ or $\displaystyle \displaystyle SE = \frac{s}{\sqrt{n}}$

Which one to use in your case? and why?

Quote:

Originally Posted by JGaraffa

c) give a point estimate of the population mean

Will be the same as the sample mean

Quote:

Originally Posted by JGaraffa

e) construct a 95% confidence interval for the mean

Try $\displaystyle \displaystyle \bar{x}\pm Z_{0.95}\times SE$