A non-probability sampling method is a sampling method in which the chance of each member of the population appearing in the sample is not known, for example, quota sampling.
A simple random sample is a sample selected in such a way that every item in the population has an equal chance of being included. If a sample is selected using random sampling, it will be free from bias (since every item will have an equal chance of being selected). Once the sample has been selected, valid inferences about the population being sampled can be made. A random sample is not necessarily a perfect sample.
If random sampling is used then it is necessary to construct a sampling frame. A sampling frame is a numbered list of all items in a population.
A sampling frame should have the following characteristics. (a) Completeness. Are all members of the population included on the list? (b) Accuracy. Is the information correct? (c) Adequacy. Does it cover the entire population? (d) Up to dateness. Is the list up to date? (e) Convenience. Is the sampling frame readily accessible? (f) Non-duplication. Does each member of the population appear on the list only once?
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