There are many methods of sampling when doing research such as;
Probability methods: This is the best overall group of methods to use as you can subsequently use the most powerful statistical analyses on the results.
Whole population is available
There are specific sub-groups to investigate (eg. demographic groupings).
When a stream of representative people are available (eg. in the street).
When population groups are separated and access to all is difficult, eg. in many distant cities.
Quota methods: For a particular analysis and valid results, you can determine the number of people you need to sample. In particular when you are studying a number of groups and when sub-groups are small, then you will need equivalent numbers to enable equivalent analysis and conclusions.
You have access to a wide population, including sub-groups
You know the population distribution across groups, and when normal sampling may not give enough in minority groups
There is likely to a wide variation in the studied characteristic within minority groups
Selective methods: Sometimes your study leads you to target particular groups
You are studying particular groups
You want expert opinion
You seek similar subjects
When sought ‘typical’ opinion may get lost in a wider study, and when you are able to identify the ‘typical’ group
You are specifically seeking differences, eg. to identify sub-groups or potential conflicts