In the April 2006 issue of MRT, Equation 2 in Tech Speak was incorrect. The correct equation appears below. MRT regrets the error.
n= Z2α/2 p(1 – p)/ d2
where zα/2 is the argument of the unit normal distribution for a confidence of 1-α and d is one-half of the confidence interval. For example, for 90% confidence, zα/2 = 1.65.