11. Comparing Proportions

# 11.2 Confidence Interval for the Difference between Two Proportions

The form of the confidence interval is

with

where, as usual you can get from the last line of the t Distribution Table.

Example 11.2 : Using the data from Example 11.1, find the 95 confidence interval for .

Solution : The relevant numbers from Example 11.1 are: , , and , , .

Compute (after finding from the t Distribution Table)

and

So

with 95 confidence. (Note that this corresponds with the rejection of in Example 11.1 since 0 is not in the confidence interval.)