Further explanation of Stratified Sampling
This could involve only interviewing those with a characteristic which is needed for the sample. For example, the producers of a 'fast-food' may choose to only interview teenage males as these could be significant buyers of the food product in the future. Within this section of the population, people could be found at random. This gives rise to the name 'stratified random sample'. Also, these teenage males could be found by setting quotas such as by 'social class'.
Source: Marcouset. A, Business Studies (2003:25)
Information on stratified sampling and see below:
Stratified Random Sampling
A sampling procedure for which the population is first divided into strata or subgroups based on designated criteria and then the sample is drawn, either proportionately or disproportionately, from each subgroup.