A research sample in which the elements are selected from a sampling frame with a known, non-zero probability of selection. Some common types of probability samples are:
- Simple random sample: Each element is selected at random from the population as a whole;
- Stratified sampling: The original sampling frame is divided into mutually exclusive sub-frames (strata) and separate and independent samples are selected from each strata.
- Multi-stage samples: Sampling elements are selected in stages. For example, the first stage might involve selecting block groups within counties. The second stage might then be a sampling of blocks. Different frames and different elements are usually used at each stage. The probabilities of selecting each element are known for each stage of selection.
- Cluster Sample: The sample elements are groups of units and not individual units. Each element is identified with only one cluster in the selection process.