Sampling Theory

Introduction 
In a survey, a researcher aimed to estimate the parameters in a population. In this case, sampling has very important role. Samples taken should be as much as possible represent the population. The sampling method is selected based on characteristics of the population. A researcher must be very aware of the state population as object in research.
Population, Elementary Unit, Frame and Sampling Unit,
In sampling theory, we need to understand in advance of the population, sampling unit, frame, and the sample. As a simple example, suppose we would estimate the average height of female students per grade 6 elementary school in town X. In that town there are 15 primary schools. In sampling, we take the 10 female students from 5 schools grade 6 randomly selected. The height of each student can be found on registration card. From this case, we can mention :
1. Population : all female students 6th grade in town X
2. Elementary Unit : each female student
3. Frame : registration card
4. Sampling unit : all of the female student selected 
 Case Study
Suppose we wish to estimate the average amount of milk a cow produces in a state B. The elementary unit is the cow and the totally of cows in state B is the population. Suppose there is no list or record of the cows in the state B, but there is a list of dairy farms. Then a procedure woukld be to use the dairy farms as a sampling unit and the list of such farms as the frame.
Sampling Method
1. Simple Random Sampling
2. Stratified Random Sampling
3. Simple Cluster Sampling