Random sampling

A simple random sample is one where every member of the population has an equal chance of being selected for the sample. This might be achieved by assigning every member of the population a number and using a random number generator to select the members of the sample.

In systematic sampling, the members of the sample are chosen at regular internals from an ordered list.

In stratified sampling, the population is divided into groups, and a random sample is selected from each.

Non-random sampling

In quota sampling, a sample is selected which reflects the characteristics of the whole population.

In opportunistic sampling, the sample is taken from members of the population who happen to be available at the time and fit the required criteria.