What are the four types of samples?

The four types of samples can be classified in several ways depending on field, but in statistics and research, a common classification of four types of sampling methods includes simple random sample, stratified sample, cluster sample, and systematic sample, all of which are probability sampling methods. Simple random sample is where every member of the population has an equal chance of being selected, like drawing names from a hat. Stratified sample is where the population is divided into subgroups or strata based on characteristics like age or gender, and random samples are taken from each stratum. Cluster sample is where the population is divided into clusters, such as geographic areas, and entire clusters are randomly selected. Systematic sample is where every nth member is selected from a list, such as every 10th person. In another classification, four types could refer to probability sampling types versus non-probability types including convenience sample, purposive sample, quota sample, and snowball sample. In laboratory medicine, four types of samples could be blood, urine, tissue, and swab samples. In quality control, four types could be random, representative, composite, and grab samples. So four types usually refers to major sampling methods used to ensure representativeness.