In random sampling,all items have an equal chance of being selected as part of the sample.Five common random sampling techniques are:simple random sampling,systematic sampling,stratified sampling,cluster sampling,and multi-stage sampling.With simple random sampling,each item in a population has an equal chance of inclusion in the sample.Systematic sampling,sometimes called interval sampling,means that there is a gap,or interval,between each selection.This method is often used in industry,where an item is selected for testing from a production line (say,every fifteen minutes) to ensure that machines and equipment are working to specification.In stratified sampling,the population is divided into groups called strata.A sample is then drawn from within these strata.Cluster sampling divides the population into groups,or clusters.A number of clusters are selected randomly to represent the population,and then all units within selected clusters are included in the sample.No units from non-selected clusters are included in the sample.They are represented by those from selected clusters.This differs from stratified sampling,where some units are selected from each group.
Multi-stage sampling is like cluster sampling,but involves selecting a sample within each chosen cluster,rather than including all units in the cluster.