Question

A random sample of 17 subjects was asked to perform a given task. The time in seconds it took each of them to complete the task is recorded below:

29,29,30,34,35,36,36,37,37,38,38,39,41,45,46,48,50

If we assume that the completion times are normally distributed, find a 90% confidence interval for the true mean completion time for this task. Then complete the table below.

A.) What is the lower limit of the confidence interval?

B.) What is the upper limit of the confidence interval?

Answer 1

Mean = 38.118

Answer 2

use the standard deviation

Answer 3

standard deviation = 6.264?

Answer 4

Okay maybe I've got this..

Lower = 27.8, Higher = 48.4?

Answer 5

Okay no, that's not it..

Answer 6

35.5 and 40.8? :(

Answer 7

im not sure if we should use the t formula

Answer 8

Statistics are what nightmares are made of. I don't get this at all.

Answer 9

I think we are supposed to use the t curve

Answer 10

my book says
" when the population st. dev is known and the variable is normally distributed, or when st. dev is unknown and n>= 30 , then we use the standard normal distribution when we find confidence intervals for the mean "

Answer 11

"In many situations the pop std dev is not known and the sample size is less than 30, in such situations we use the sampl std . dev in place of the pop. st. dev and the t distribution"