Choosing a sampling method
There are many methods of sampling when doing research. This guide can help you choose which method to use. Simple random sampling is the ideal, but researchers seldom have the luxury of time or money to access the whole population, so many compromises often have to be made.
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.
| Method | Best when |
| Simple random sampling | Whole population is available. |
| Stratified sampling (random within target groups) | There are specific sub-groups to investigate (eg. demographic groupings). |
| Systematic sampling (every nth person) | When a stream of representative people are available (eg. in the street). |
| Cluster sampling (all in limited groups) | 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.
| Method | Best when |
| Quota sampling (get only as many as you need) | You have access to a wide population, including sub-groups |
| Proportionate quota sampling (in proportion to population sub-groups) | You know the population distribution across groups, and when normal sampling may not give enough in minority groups |
| Non-proportionate quota sampling (minimum number from each sub-group) | 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.
| Method | Best when |
| Purposive sampling (based on intent) | You are studying particular groups |
| Expert sampling (seeking 'experts') | You want expert opinion |
| Snowball sampling (ask for recommendations) | You seek similar subjects (eg. young drinkers) |
| Modal instance sampling (focus on 'typical' people) | When sought 'typical' opinion may get lost in a wider study, and when you are able to identify the 'typical' group |
| Diversity sampling (deliberately seeking variation) | You are specifically seeking differences, eg. to identify sub-groups or potential conflicts |
Convenience methods
Good sampling is time-consuming and expensive. Not all experimenters have the time or funds to use more accurate methods. There is a price, of course, in the potential limited validity of results.
| Method | Best when |
| Snowball sampling (ask for recommendations) | You are ethically and socially able to ask and seek similar subjects. |
| Convenience sampling (use who's available) | You cannot proactively seek out subjects. |
| Judgment sampling (guess a good-enough sample) | You are expert and there is no other choice. |
Ethnographic methods
When doing field-based observations, it is often impossible to intrude into the lives of people you are studying. Samples must thus be surreptitious and may be based more on who is available and willing to participate in any interviews or studies.
| Method | Best when |
| Selective sampling (gut feel) | Focus is needed in particular group, location, subject, etc. |
| Theoretical sampling (testing a theory) | Theories are emerging and focused sampling may help clarify these. |
| Convenience sampling (use who's available) | You cannot proactively seek out subjects. |
| Judgment sampling (guess a good-enough sample) | You are expert and there is no other choice. |
Reference
http://changingminds.org/explanations/research/sampling/choosing_sampling.htm
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