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Choosing a sampling method

Techniques > Research > Sampling > 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.