Step 1:

The 95 % confidence interval, .

The is Z-score value for the area of the region of value .

Look into inverse standard normal distribution table.

Now the value of Z-score for is .

Consider only positive values of Z.

Hence for 95 % confidence interval is .

Solution:

for 95 % confidence interval is .

Step 1:

Definition of specific confidence interval for a proportion : .

where is the population proportion.

is the sample proportion = 28 % of women who purchase books online.

n is the sample size = 427 women.

is the confidence interval.

Substitute and in the above formula.

This is how .

Conclusion:

5% of confidence interval for the proportion of all 29 % women purchase books online is .