Question

The Highway Safety Department wants to study the driving habits of individuals. A sample of 81 cars traveling on the highway revealed an average speed of 67 miles per hour with a standard deviation of 9 miles per hour.

a. Compute the standard error of the mean.

b. Determine a 99% confidence interval estimate for the speed of all cars.

Answer 1

solution:-

given that mean = 67 , standard deviation = 9 , n = 81

degree of freedom df = n - 81 - 1 = 80

we look into t table with df and with 99% confidence level

critical value t = 2.639

a. standard error

=> 9/sqrt(81) = 1

b. confidence interval

=> mean +/- t * standard error

=> 67 +/- 2.639*1

=> (64.361 , 69.639)