Some of the important methods of selecting the sample are:
I. Random sampling
II. Interval sampling or systematic sampling
I. Random Sampling: This selection ensures that all items in the population or within each stratum have a known chance of selection. Random sampling includes the following important methods:
(1) Simple random sampling: It is considered that random number tables are simple and easy to use and also provide assurance that the bias does not affect the selection. Under this method each unit of the whole population e.g. purchase or sales invoice has an equal chance of being selected. For example, the population can be considered homogeneous, if say, debtors balance fall within the range of Rs. 5,000 to Rs. 25,000 and not in the range between Rs. 25 to Rs. 250,000.
(2) Stratified Sampling: This method involves dividing the whole population to be tested in a few separate groups called strata and taking a sample from each of them. The number of groups into which the whole population has to be divided is determined on the basis of auditor judgement. For example: debtor balance of a company is divided into the following four groups:
• Balance in excess of Rs. 100,000;
• Balance in the range of Rs. 75,000 to Rs. 100,000;
• Balance in the range of Rs. 25,000 to Rs. 75,000; and
• Balance below the range of Rs. 25,000.
II. Interval sampling or systematic sampling: It involves selecting items using a constant interval between selections, the first interval having a random start. The interval might be a certain number of items or monetary totals. When using systematic selection, the auditor should determine the population is not structured in such a manner that the sampling interval corresponds with a particulars pattern in the population. Systematic sampling includes the following important methods:
(1) Block sampling: This method involves the selection of a defined block of consecutive items. For example, take the first 200 sales invoice for the sales day book in the month of July.
(2) Cluster sampling: This method involves dividing the population into group of items known as clusters. A number of clusters are randomly selected from all the clusters rather than individual items of the population.