A simple random sample is the simplest form of probability sampling and should be used when each population requires an equal chance of selection; for example when a study of the number of people that may purchase a DVD player for Christmas is required.
A cluster sample is used when we require need a study in depth (case study). This is typically used when we want to minimize costs and we can have a heterogeneous “cluster” per area.
Stratification is utilized when we want to study sub-groups. It can be used well when we are going to use groups that are homogenous within and heterogeneous to each other. It also increases a sample’s statistical efficiency.