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Sampling is the process of selecting a sample from the population. The probability sampling uses a random process to ensure that every member of the population has a known non-zero probability of being included in the sample. The probability sampling methods include simple random sampling (every sample of same size has same chance of being selected and every sample unit has same chance of being selected), systematic sampling (selecting elements by periodic process), stratified random sampling (the population is divided into strata according to characteristics of interest, then simple random sample is selected from each stratum) and cluster sampling (the population is first divided into clusters of homogenous units usually based on geographical contiguity, a sample of each cluster is then selected and all units in selected clusters are studied).