In: Statistics and Probability
Efficiency experts study the processes used to manufacture items in order to make them as efficient as possible. One of the steps used to manufacture a metal clamp involves the drilling of three holes. In a sample of 70 clamps, the mean time to complete this step was
57.8 seconds. Assume that the population standard deviation is =σ 10
seconds. Round the critical value to no less than three decimal places
Construct a 90% confidence interval for the mean time needed to complete this step.
Solution :
Given that,
Point estimate = sample mean = = 57.8
Population standard deviation =
= 10
Sample size = n =70
At 90% confidence level the z is
= 1 - 90% = 1 - 0.90 = 0.1
/ 2 = 0.1 / 2 = 0.05
Z/2 = Z0.05 = 1.645 ( Using z table )
Margin of error = E = Z/2
* (
/n)
= 1.645 * (10 / 70
)
= 1.97
At 90% confidence interval mean
is,
- E < < + E
57.8 - 1.97 <
< 57.8 + 1.97
55.83 <
< 59.77