A sampling technique in which units are randomly sampled from a population that has been divided into categories is called

Stratified random sampling divides the population into categories, then samples units randomly within each.

The strata are internally homogeneous on the characteristic under study.

Simple random sampling draws from the whole population without any prior grouping.

Cluster sampling divides the population into groups and samples whole clusters.

Systematic sampling selects every kth unit from an ordered list.

Stratification improves precision when strata differ markedly from one another.