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:

    1. Simple random sampleEach element is selected at random from the population as a whole;
    2. Stratified samplingThe original sampling frame is divided into mutually exclusive sub-frames (strata) and separate and independent samples are selected from each strata.
    3. 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.
    4. Cluster SampleThe sample elements are groups of units and not individual units. Each element is identified with only one cluster in the selection process.