Plan sample size for a proportion

Use n=z²p*(1-p*)/E², with a planning proportion p*. If no prior value is available, p*=0.5 maximises p*(1-p*) and gives a conservative value. Round up. For the same confidence and variability, margin is proportional to 1/√n: quartering the margin requires 16 times the sample size. Sample-size planning should consider cost, heterogeneity, resources, purpose and precision.