Quality management 385

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Q 1 - The tensile strength of fiber used in manufacturing cloth is of interest to the purchaser. Previous experience indicates that the standard deviation of tensile strength is 2 psi.

A random sample of eight fiber specimens is selected, and the average tensile strength is found to be 127 psi.

a ) Build a 95% lower confidence interval in the mean tensile strength.

b) What can you conclude?

Q 2 - Payment times of 100 randomly selected customers this month had an average of 35 days. The standard deviation from this group was 2 days.

A ) Build a 90% two-sided confidence interval on the mean payment time.

B ) Build a 99% two-sided confidence interval on the mean payment time.

C ) Is it possible that the average time is 30 days?

Q 3 - Two machines are used for filling glass bottles with a soft drink beverage. The filling processes have known standard deviations 1 =0.010 liter and 2 =0.015 liter, respectively. A random sample of n 1 =25 bottles from machine 1 and n 2 =20 bottles from machine 2 results in average net contents of 2.04 liters and 2.07 liters from machine 1 and 2, respectively.

Test the hypothesis that both machines fill to the same net contents using a 95% two-sided confidence interval on the difference of fill volume. What are your conclusions?