### Definition

A **probability sample** is a sample in which each population element has a known, nonzero chance of being included.

A **simple random sample** is a probability sample in which each population element has a known and equal chance of being included in the sample and in which every combination of *n* population elements is a sample possibility and is just as likely to occur as any other combination of *n* units.

A **cluster sample** is a probability sample distinguished by a two-step procedure in which 1) the parent population is divided into mutually exclusive and exhaustive subsets, and 2) a random sample of subsets is selected. If the investigator then uses all of the population elements in the selected subsets for the sample, the procedure is **one-stage cluster sampling**. If a sample of elements is selected probabilistically from the subsets, the procedure is **two-stage cluster sampling**.

**Area sampling** is a form of cluster sample in which areas (i.e. census tracts, blocks) serve as the primary sampling units. The population is divided into mutually exclusive and exhaustive areas using maps, and a random sample of areas is selected. If all the households in the selected areas are used in the study, it is **one-stage area sampling**. If the areas are sub-sampled with respect to households, the procedure is **two-stage area sampling**.

A **stratified sample** is a probability sample that is distinguished by the two-step procedure in which 1) the parent population is divided into mutually exclusive and exhaustive subsets, and 2) a simple random sample of elements is chosen independently from each group or subset.

A **systematic sample** is a probability sample in which every *kth* element in the population is designated for inclusion in the sample after a random start.

### See Also

Sample representativeness

Sampling

Non-probability sampling

### References

- American Marketing Association,
*AMA Dictionary.*