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The auditor should select sample items in such a way that the sample can be expected to be representative of the population. This requires that all items in the population have an opportunity of being selected. While there are a number of selection methods, three methods commonly used are:

Random selection, which ensures that all items in the population have an equal chance of selection, for example,
by use of random number tables.

Systematic selection, which involves selecting items using a constant interval between selections, the first interval having a random start. The interval might be based on a certain number of items (for example, every 20th voucher number) or on monetary totals (for example, every ` 1,000 increase in the cumulative value of the population). When using systematic selection, the auditor would need to determine that the population is not structured in such a manner that the sampling interval corresponds with a particular pattern in the population. For example, if in a population of branch sales, a particular branch’s sales occur only as every 100th item and
the sampling interval selected is 50, the result would be that the auditor would have selected all, or none, of the sales of that particular branch.

Haphazard selection, which may be an acceptable alternative to random selection, provided the auditor attempts to draw a representative sample from the entire population with no intention to either include or exclude specific units. When the auditor uses this method, care needs to be taken to guard against making a selection that is biased, for example, towards items which are easily located, as they may not be representative.

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