Sampling is the procedure for choosing a sample from a defined statistical population. The population from which a sample is drawn is the sampling frame and the ratio between the size of the sample and the size of the parent population is the sampling function.
Of the sampling methods developed purposive sampling and probability sampling are general types. In purposive sampling the samples drawn are considered to be typical of the population as a whole.
The researchers choose sample randomly and subjectively on the basis of convenience. This kind of sampling does not ensure that every individual or unit has an equal chance of being included and so is likely to be biased. In probability sampling rigorous statistics methods are used to select samples representative of the parent population. In random sampling the choice of individuals or units is left entirely to chance. In this the selection of the particular individual does not affect the chance of selection of any other individual. In systematic (grid or regular) sampling the choice is made in a regular way after selecting a random sampling point. It is quicker and easier than random sampling.