ISA 530 also requires the auditor to select items for the sample in such a way that each sampling unit in the population has a chance of selection. When statistical sampling is used, each sampling unit has a known probability of being selected. When non-statistical sampling is used, judgement is applied. However, it is important that the auditor selects a representative sample, free from bias, by choosing sample items that have characteristics typical of the population. The main methods of selecting samples are random selection, systematic selection and haphazard selection. We discuss these and other methods below.