Question

A quality-conscious disk manufacturer wishes to know the fraction of disks his company makes which are defective.

Step 1 of 2:

Suppose a sample of 425 floppy disks is drawn. Of these disks, 387 were not defective. Using the data, estimate the proportion of disks which are defective. Enter your answer as a fraction or a decimal number rounded to three decimal places.

Step 2 of 2:

Suppose a sample of 425 floppy disks is drawn. Of these disks, 387 were not defective. Using the data, construct the 85% confidence interval for the population proportion of disks which are defective. Round your answers to three decimal places.

Answer 1

step 1:  proportion of disks which are defective =(425-387)/425 =0.089

step 2:

std error       =Se		=√(p*(1-p)/n) =		0.0138
for 85 % CI value of z=				1.44
margin of error E=z*std error                            =				0.020

85% confidence interval for the population proportion of disks which are defective =sample proportion -/+ margin of error

=(0.069 , 0.109)