Sampling methods

Researchers use various different approaches to identifying the people they want to include in research. Here is a list of what those methods are, and why they might be used:

Probability sampling (random sampling):

• People are randomly chosen from a population
• Each person in the population has the same chance of being chosen
• If the sample is large enough, you can be confident that there is a good chance the rest of the population will behave in the same way

Stratified random sampling: 

• Sometimes researchers are interested in understanding more about the specific sub-groups within populations, such as different ethnic groups or age groups
• In stratified random sampling, researchers select groups (or 'strata') and randomly choose participants from within those groups
• This method ensures the sample contains enough people from each group that the researchers are interested in, which allows researchers to study differences within and between those group

Geographic cluster sampling:

• If a population is spread across a large geographical area, like a large city or country, it might be easier to use cluster sampling than to sample from the whole population
• The population is divided into areas called clusters, and researchers randomly select which clusters to include in the study
• Everyone in each cluster is asked to take part in the research, so the sample represents the diversity of different people within the each area
• Cluster sampling is a quicker and easier way to get a representative sample, but there is a higher chance of error than with probability sampling

Panel sampling: 

• Panel sampling involves randomly choosing a group of people to be part of a panel that takes part in a study several times over a period of time
• For example, in a longitudinal survey, the same panel of people might be surveyed repeatedly over time
• Panel samples allows researchers to study changes within the population as well as changes in individual people, however they can be vulnerable to attrition if people leave the study before it is finished

Cohort sampling:

• Cohort sampling involves recruiting from a group (or 'cohort') of people who share a specific event, such as the year they were born
• Both cohort sampling and panel sampling are used to study changes over time, but they are not exactly the same
• While studies that use panel sampling follow the same groups of individuals, studies that use cohort sampling follow a cohort, but not necessarily the same individuals every time

Quota sampling: 

• This is a non-random form of sampling and is often used when there is little time to recruit people for a study.
• First researchers identify important characteristics that they want their sample to contain
• Then researchers set out to recruit certain numbers of participants with these characteristics. These numbers are their 'quota' for each characteristic.
• This may mean that researchers will send out more study invitations to some groups than others, if numbers of people with that characteristic are low (i.e. if a researcher is surveying people on the street and is looking for equal numbers of men and women, they may stop approaching men once their quota of male participants has been filled.
• As the sample is not randomly selected the researchers could introduce selection bias into their choices (e.g. they may not like the look of a candidate for the quota and pass them by until they find someone they do like).

Census: 

• In a census, information or measurements are collected from an entire population of a country or an area. It covers everyone in the population at the same time and asks the same core questions.
• They tend be carried out regularly, for example the UK national census is carried out every 10 years.
• Because everyone in the population is sampled, representativeness is not an issue.