Question

Schadek Silkscreen Printing, Inc., purchases plastic cups on which to print logos for sporting events, proms, birthdays, and other special occasions. Zack Schadek, the owner, received a large shipment this morning. To ensure the quality of the shipment, he selected a random sample of 440 cups. He found 28 to be defective.

a. What is the estimated proportion defective in the population? (Round the final answer to 3 decimal places.)

Estimated proportion defective           

b. What are the endpoints of a 80% confidence interval for the proportion defective. (Round the final answers to 3 decimal places.)

Endpoints    ,

c. Zack has an agreement with his supplier that he is to return lots that are 10% or more defective. Should he return this lot?

Answer 1

(a) Estimated proportion defective = 28 / 440 = 0.064
_______________________________

(b) 80% CI for proportion

The Z critical (2 tail) for = 0.2, is 1.282

The Confidence Interval is given by ME, where

The Lower Limit = 0.064 - 0.015 = 0.049

The Upper Limit = 0.064 + 0.015 = 0.079

The 80% Confidence Interval is (0.049 , 0.079)

____________________________________

(c) No, Jack should not return this lot as the true amount of defectives lie between 0.049 (4.9%) and 0.079 (7.9%), which are less than 10%.

_____________________________________