Question

A quality control engineer is interested in the mean length of sheet insulation being cut automatically by machine. The desired length of the insulation is 12 metres. It is known that the standard deviation in the cutting length is 0.15 metres. A sample of 144 cut sheets yield a mean length of 12.14 metres. This sample will be used to obtain a 90% confidence interval for the mean length cut by machine. What are the two limits of the confidence interval?

Answer 1

Mean = 12.14 Meters

Sample Size = 144

Standard deviation = 0.15 Meters

Z Value at 90% interval = 1.645

Upper Limits = Means + Z * SD / Sqrt(Sample Size)

Upper Limits = 12.14 + 1.645 * 0.15 / Sqrt(144)

Upper Limits = 12.16

Lower Limits = Means - Z * SD / Sqrt(Sample Size)

Lower Limits = 12.14 - 1.645 * 0.15 / Sqrt(144)

Lower Limits = 12.12

Confidence Interval = (12.12,12.16)