Question

A random sample of 13 students were asked how long it took them to complete a certain exam. The mean length of time was 111.5 minutes, with a standard deviation of 83.0 minutes. Find the lower bound of the 90% confidence interval for the true mean length of time it would take for all students to complete the exam.

Answer 1

Solution :

degrees of freedom = n - 1 = 13 - 1 = 12

t/2,df = 1.356

Margin of error = E = t/2,df * (s /n)

= 1.356 * ( 83.0 / 13)

Margin of error = E = 23.02

The 99% confidence interval estimate of the population mean is,

- E  

111.5 - 23.02

= 88.48

lower bound : 88.48