Can You Compare And Contrast Stratified Sampling And Cluster Sampling?

When the population to be studied is not homogeneous with respect to variable or characteristics under study then it is divided into small homogeneous groups called strata. Items with in each stratum are homogeneous with respect to characteristics under study. From each stratum a simple random sample is selected in proportion to the stratum size and overall sample is obtained by combining the samples for all strata. A cluster is a collection of units found in a nature for example, if we are considering industrial establishment, then an industrial factory will be cluster. Clusters are called primary sampling units.