Sampling can simply be defined as an act of drawing sample units from the overall population in order to make it easier to give inferences about the whole population just by studying those units. The technique of selecting a sample plays a vital role in sampling theory and the technique to be used fully depends upon the nature of investigation. We can classify the commonly used procedures of sampling as:

- Probability sampling
- Non-probability sampling
- Mixed sampling

**Probability sampling:**The sampling in which the units are selected from the population on the basis of known probabilities is called probability sampling. The examples of probability sampling are: Simple Random Sampling, Probability Proportional to Size etc.**Non-probability sampling:**Non-probability sampling is the sampling where discretion is used to select representative units from the population or to infer that a sample is a representative of the whole population. This method is not generally used because it gives rise to bias and prejudice of the enumerator. However, in case the enumerator is experienced in the work, this technique can give proper results. For example, the samples of a market research survey of the performance of a company’s new car would be the purchasers of that new car.**Mixed sampling:**In mixed sampling, the samples are selected partly on the basis of probability and partly on the basis of a fixed sampling rule. Such samples are called mixed samples.