Sampling: Selection of Participants

What is the target population?

The target population is a group of people that share a given set of characteristics about which a researcher wishes to draw conclusions (e.g. all football fans in the UK). However a target population is usually too large for each individual to be investigated so a sample, which in a subset of the population, is obtained. The sample should be representative, meaning that the participants included should share important characteristics with the target population to enable findings to be generalised.

What is the ‘random’ sampling method?

This is where every member of the population has an equal chance of being selected. The easier way to do this is to place all the names( or allocate numbers) from a target population into a hat and draw out the required sample. Computer programmes can also generate random lists of participants in an unbiased way.

What is the ‘stratified’ sampling method?

Subgroups (or strata) within a population are identified. Participants are obtained from each of the strata in proportion to their occurrence in the population. Selection from the strata is done using a random technique.

What are the strengths of ‘stratified’ sampling? (1)

• By guaranteeing that all key characteristics of the population are present in the sample, it not only avoids investigator bias afterwards, but it is also representative and can be generalised.

What are the limitations of ‘stratified’ sampling? (3)

• If not all key features of the population are not identified then the sample may not be representative, thus limiting generalisability.

• Very time consuming to identify subgroups and then select participants randomly.

• Need to know all the names of the target population.

What is the ‘opportunity’ sampling method?

This is the most common method used to select a sample. Approach anyone in the immediate area who happens to be available to ask to participate, e.g. asking people in the street or friends in college.

What is a strength of the ‘opportunity’ sampling?

• The easiest method as you use the first participants that you find, which means it takes less time to locate your sample than the other methods.

What are the limitations of the ‘opportunity’ sampling? (2)

• Unlikely to be representative of the target population as the sample is taken from a small part of the target population

• The researcher may consciously or unconsciously show bias in the sample selection, thus limiting generalisability.

What is the ‘volunteer’( self- selected) sampling method?

Advertisements are used to attract participants. The participants then volunteer themselves. Usually requires an incentive (payment/prize).

What is the strength of the ‘volunteer’ sampling?

• This is an way method to use with less initial work involved than in random sampling.

What is the limitation of the ‘volunteer’ sampling?

• Sample bias is likely as only certain types of people are likely to volunteer. They are atypical respondents with characteristics not necessarily common of the population (e.g. highly motivated, time on their hands), this limits the generalisability.