A finite subset of the population, selected from it with the objective of investigating its properties is called a sample and the number of units in the sample is known as the sample size.
Sampling is a tool which enables us to draw conclusions about the characteristics of the population after studying only those objects or items that are included in the sample. The main objectives of the sampling theory are:
(i) To obtain the optimum results, i.e., the maximum information about the characteristics of the population with the available sources at our disposal in terms of time, money and manpower by studying the sample values only.
(ii) To obtain the best possible estimates of the population parameters.