Probability sampling

A sampling method in which every population unit has a known, nonzero chance of selection. It supports design-based estimates of population characteristics and their sampling uncertainty.

Sampling frame: A list or other operational representation of population units from which a sample can be selected. It identifies the units whose selection probabilities a sampling design can specify.
Target population: The complete set of units about which a study seeks to draw conclusions. Selection probabilities are meaningful only relative to a defined set of units.
Survey sampling: The theory and practice of selecting samples to collect information about a population. Probability sampling is a central approach to designing population surveys.
Nonprobability sampling: A sampling approach in which population units’ selection probabilities are unknown or not specified by a random design. It lacks the known selection probabilities that ground design-based inference here.
Simple random sampling: A design that selects a fixed number of units so every possible sample of that size is equally likely. It is the basic equal-probability design within probability sampling.
Sampling unit: The unit selected at a particular stage of a sampling design. Multistage designs can select different units at successive stages.
Weighting (statistics): A statistical procedure that assigns different influence to observations when calculating estimates. Survey weights commonly reflect inverse selection probabilities and adjustments to the sample.
Convenience sampling: A nonprobability method that selects units because they are readily available. Availability, rather than a probability mechanism, determines inclusion.
Stratified sampling: A design that divides a population into strata and samples separately within each stratum. Separate stratum-level selection can improve precision and ensure subgroup representation.
Sample size: The number of units included in a study sample. Sample size affects precision alongside the design and selection probabilities.

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