Stratified Sampling is one of the methods of Random Sampling. This method involves dividing the whole population to be tested in a few groups called strata and taking a sample from each of them. Each stratum is treated as if it were a separate population and if proportionate items are selected from each of the stratum. The group into which the whole population is divided is determined the auditor on the basis of his judgment, e.g. entire expense vouchers may be divided into:
(i) Vouchers above Rs.100,000
(ii) Vouchers between Rs.25, 000 and Rs.100,000
(iii) Vouchers below Rs.25, 000
The auditor can then decide to check all vouchers above Rs.1, 00,000, 50% between 25,000 and Rs.100,000 and 25% of those below Rs.25, 000.
The reasoning behind the stratified sampling is that for a highly diversified population, weights should be allocated to reflect these differences. This is achieved selecting different proportions from each stratum. It can be seen that the stratified sampling is simply an extension of simple random sampling.