Statistics

Individuals are chosen as opportunities arise

Individuals are chosen through random procedure

if populations are divided into representative subgroups, then you take samples from a few of these subgroups.

If the parent population is divided into distinct subgroups (strata) that clearly represent underlying features, then these strata may affect the outcome being investigated. In which case, one can sample based on strata so all strata are to some extent represented. We can either sample equal amounts of people from each strata, or we can sample strata based upon their prominence in the complete population

We require a set number of people per strata, with no reason why we are sampling in such a way

Volunteers are sampled

any identifiable method of sampling