Summary Employing a Wald confidence interval to test hypotheses about population proportions could lead to an increase in Type I or Type II errors unless the hypothesized value, p0, is used in computing its standard error rather than the sample proportion. Whereas the Wald confidence interval to estimate a population proportion uses the sample proportion in computing its standard error, hypothesis tests about a population proportion should use the hypothesized value, p0, in its standard error. As a result, using the Wald confidence interval to test hypotheses about a population proportion could lead to an increase in either a Type I or a Type II error.