MS 150 Statistics Quiz 07 Fall 2003 8.1 • Name: _______________
Over a sample size n of 30 weeks the cafeteria found that they have been using
a sample mean x
of 22.5 bags of rice per week with a sample standard deviation sx of 2.5. Construct a 95% confidence interval for the population mean number of rice bags.
 _______ Write down the sample size n.
 _______ Is the sample size n greater than or equal to 30?
 _______ Write down the sample sample mean
x.
 _______ Write down the sample standard deviation sx.
 _______ Write down the confidence level c.
 Use the information above to sketch the 95% confidence interval:
 _______ Determine the area in both tails.
 _______ Determine the area in the left tail.
 _______ Determine the left z_{c} value.
 _______ Write down the right z_{c} value.
 _______ Determine the error E.
 _______ Determine the left (lower) bound for the population mean µ.
 _______ Determine the right (upper) bound for the population mean µ.
 The confidence interval for the population mean number of rice bags µ is:
_________ < µ < _________
Confidence interval statistics 
Find a z_{c} value from a confidence level c 
z_{c} 

=ABS(NORMSINV((1c)/2)) 
Calculate an error tolerance E of a mean for n >= 30 using
sx 
E 

=z_{c}*sx/SQRT(n) 
Calculate a confidence interval for a population mean µ from
a sample mean x and an error tolerance E 

xE<= µ
<=x+E 
