Question

what is a plus four confidence interval?

Answer 1

The plus four method we add two successes and two failurer in each sample. Suppuse we have a sample of size n with x number of successes.

Using plus four method we have

So, for eaxmple, for 95% confidence interval so critical value of z will be

Confidence interval for population proportion will be

This simple adjustment is used to make more accurate confidence intervals. Generally it is used when numebr of successes or number of failures are very small (that is less than 5).