How do sample surveys collect data for statistical studies?

Sample surveys collect data through various methods designed to obtain a representative subset of a population for statistical inference.1

Data Collection Methods

• Questionnaires: The most common approach involves mail, telephone, or personal interviews. These rely on carefully phrased and sequenced questions.1
• Direct Observation: Some surveys, such as auditing accounting records or sampling large databases, collect data by directly observing units rather than using questionnaires.1

Sampling Strategies

Statisticians prioritize probability sampling, where each unit has a known chance of selection, allowing for the calculation of sampling error. Common techniques include:1
* Simple random sampling: Every possible sample of a given size has an equal probability of selection.12
* Stratified simple random sampling: The population is divided into homogeneous groups (strata), and samples are drawn from each.1
* Cluster sampling: The population is divided into heterogeneous groups (clusters). Researchers select clusters and collect data from all or a sample of units within them.1

Conversely, nonprobability sampling relies on convenience or judgment. While faster and cheaper, it lacks an objective basis for measuring error and should be used with extreme caution.1