The sampling which is done randomly is known as random sampling and we can calculate the probability of this sample. When the sample is not chosen randomly and we cannot calculate its probability, then it is called non-probability sampling. There are four types of non-probability sampling that are purposive sampling, quota sampling, snowball sampling and dimensional sampling. Purposive sampling and quota sampling methods are most commonly used. A purposive sampling is a non-random sample in which the selection of the sample is based on a person's expertise about the population. Quota sampling may be regarded as a special form of stratified sampling, It is generally used in marketing service.

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