Auditing Sampling
Sampling is a very important issue in auditing. It is necessary when there are so many records in a population that an auditor cannot hold a detailed examination of each record. In that case an auditor tests less than 100 percent of an entire population and then makes a conclusion about the whole. Sampling greatly reduces costs and time of auditing and very often it turns out to be more expedient than the total inspection. It is efficient when a population possesses many similar transactions (for example while checking accounts payable). At the same time an auditor must keep in mind the risk that the sample will not represent the true story of the rest of the population; and if the sample is too small the risk increases.
Sometimes an auditor might want to examine the population without sampling. This can happen if it is very small, so that it can be easily studied. Also if an auditor finds an error in the sample, examined by him, he can check the entire population to prevent missing other mistakes. When an auditor wants to make sure a bunch of figures adds up or that a bunch of income and expenses balance to zero, he cannot just take a random sample to see that. An auditor might want to check all cash documents, securities and sums paid out on account without sampling because he would want to be very thorough when investigating money. 
Bibliography
1. Janet L. Colbert. (2001, February). Audit Sampling.
Internal Auditor. From LookSmart Taxes, website http://www.looksmarttaxes.com/p/articles/mi_m4153/is_1_58/ai_71268474