where z 1⁄4 F 1 ð1 a=2Þ is the 100ð1 a=2Þth percentile of the standard normal distribution and q^ 1⁄4 1 p^ . However, the Wald interval has poor performance (see Vollset, 1993; Agresti and Coull, 1998; Newcombe, 1998; Brown et al., 2001, 2002). There is a simple continuity corrected Wald interval (Snedecor and Cochran, 1967)