Question

It is known that the amount of time needed to change the oil on a car is normally distributed with a standard deviation of 5 minutes. The amount of time to complete a random sample of 10 oil changes was recorded and listed here. Compute the 99% confidence interval estimate of the mean of the population.

11 10 16 15 18 12 25 20 18 24

Answer 1

(11.605 ; 22.195)

Explanation:

11 10 16 15 18 12 25 20 18 24

Sample mean, xbar = Σx / n = 169 / 10 = 16.9

Standard deviation = 5.152 ( using calculator)

Confidence interval :

Xbar ± Margin of error

Margin of Error = Tcritical * s/√n

Df = n - 1 = 10 - 1 = 9

Tcritical at 99%, df =9 = 3.2498

Margin of Error = 3.2498 * 5.152/√10 = 5.2946

Lower boundary :

16.9 - 5.2946 = 11.6054

Upper boundary :

16.9 + 5.2946 = 22.1946

(11.605 ; 22.195)