An employment agency wants to examine the employment rate in a city. The employment agency divides the population into the following subgroups: age, gender, graduates, nongraduates, and discipline of graduation. The employment agency then indiscriminately selects sample members from each of these subgroups. This is an example of:

A. simple random sampling.
B. purposive sampling.
C. convenience sampling.
D. stratified random sampling

In this example, the employment agency is dividing the population that it will study into different subgroups (age, gender, graduates, nongraduates, and discipline of graduation). These subgroups are known as strata. Afterwards, the agency selects sample members of each of these subgroups. As the samples are chosen on a proportional way based on subgroups, this is known as stratified random sampling.