A systematic selection of the sample items is made based on a fixed sampling interval. For example, assume that a sample size (n) of 50 items of retail inventory are to be selected from a population (N) of, say, 556 items. The sampling interval is equal to N/n or 11 (ignoring fractions). The first item chosen in the population is determined by reference to random number tables (based on the first random number selected between 1 and 11) and then after that, every 11th item is selected.
If this method is used, auditors ensure that the listing of the items in the population is not ordered in some way, otherwise the selection of items may be biased. For example, if the payroll records were organized in groups of 11 with every 11th record being the supervisor for the group, then in the above example, either (i) only the payroll records for supervisors will be selected for examination or (ii) the records for supervisors will never be selected for examination, depending on the starting number.
Audit testing procedures, index
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