Question

An educational psychologist wishes to know the mean number of words a third grader can read per minute. She wants to make an estimate at the 80% level of confidence. For a sample of 840840 third graders, the mean words per minute read was 40.6. Assume a population standard deviation of 4.1. Construct the confidence interval for the mean number of words a third grader can read per minute. Round your answers to one decimal place.

Answer 1

Solution :

Given that,

Point estimate = sample mean = = 40.6

Population standard deviation =    = 4.1

Sample size = n = 840

At 80% confidence level

= 1 - 80%  

= 1 - 0.80 =0.20

/2 = 0.10

Z/2 = Z0.10 = 1.282

Margin of error = E = Z/2 * ( /n)

= 1.282 * ( 4.1 /   840)

= 0.2

At 80% confidence interval estimate of the population mean is,

  ± E

40.6 ± 0.2

( 40.4, 40.8 )