Sampling

Specification: Sampling: the difference between population and sample; sampling techniques including: random, systematic, stratified, opportunity and volunteer; implications of sampling techniques, including bias and generalisation.

Sampling involves selecting participants from a target population. The target population is the particular  subgroup to be studied, and to which the research findings will be generalised. A target population is usually too large to study in its entirety, so sampling techniques are used to choose a representative sample.

 

For example, a sample could be 20 A‐level students from a school that has 500 A‐level students in total.

Sampling techniques

Psychologists use sampling techniques to choose people to represent the target population. If the sample is representative then psychologists can generalise the results to the target population with more credibility.

 

There are five common types of sampling:

Random sampling

With random sampling, every member of the target population has an equal chance of being selected. This involves identifying everyone in the target population and then selecting the number of participants you need in a way which gives everyone an equal chance of being selected, such as pulling names from a hat, or using a computer software package which generates names/number randomly and without bias.

 

If a researcher was trying to achieve a random sample from 500 A‐level students in a school, they would place the name of each student on role into a hat/computer name generator and then select the first 20, for example, to be participants in their study.

Evaluation of random sampling

A strength of obtaining a random sample is that it is free from researcher bias. Since the sample is generated by a computer generator or by selecting names from a hat the researcher does not have any input into who is selected. This significantly reduces the possibility of them choosing a biased sample of participants who would serve to support their aims. This means that the sample is likely to be representative so can be generalised to the target population.

 

There are drawbacks associated with the random sampling procedure. Ensuring that everyone in the target population has an equal chance of being selected is a difficult and time consuming task. It is also a possibility that individuals who are picked may be unwilling to take part. This results in the sample being more akin to a volunteer sample.

Systematic sampling

With systematic sampling, a predetermined system is used to select participants.

For example, every fifth person is chosen and the same interval is then consistently applied to the whole of the target population such as the 10th, 15th, 20th person and so on.

 

If a systematic sample of 500 A‐level students in a school was required, a researcher would list every student on role against a number, perhaps listed in alphabetical order, and then chose every 10th person to achieve a sample of 20 participants for their study (e.g. person 10, 20, 30, etc.)

Evaluation of systematic sampling

An advantage of using a systematic sampling system is that it is free from researcher bias. Since the researcher is not selecting participants by choice, but by following a predetermined system, this reduces any potential influence that the investigator may have over obtaining the sample.

 

However, the systematic sampling method may not be truly unbiased. It might be that every Nth person has a particular characteristic in common, for example being right‐handed. Although it would be fairly unlikely and unlucky to get a sample who were all similar on a particular trait, it remains a possibility with using this technique. Therefore, the sample generated may not be representative meaning generalisation to the target population would be more difficult.

Stratified sampling

In stratified sampling, subgroups within a population are identified. Participants are obtained from each stratum (‘layer’ or category) in proportion to their occurrence within the population.

 

For example, if a class of A‐level psychology had 20 students: 18 males and 2 females, and a researcher wanted a sample of 10 to participate in their study, the sample would consist of 9 males and 1 female, to represent this population proportionally.

Evaluation of stratified sampling

A strength of obtaining a stratified sample is that it is largely free from researcher bias. In this technique, the sample is generated randomly once the subcategories/strata have been identified. This significantly reduces the possibility of the researcher choosing a biased sample of participants who would serve to support their aims. This means that sample is likely to be representative because each particular subgroup, if selected appropriately, will be represented within the sample. This means that any findings generated from research with a stratified sample can be generalised to the target population with greater confidence.

 

There are limitations associated with the stratified sampling method. Ensuring that the subgroups/strata in target population are all accurately identified is sometime a difficult and time‐consuming task. Furthermore, stratification is not a perfect process since the subgroups identified cannot possibly reflect all the individual differences that exist between those in the target population. Therefore, a truly representative sample would be extremely difficult to obtain using this technique.

Opportunity sampling

Opportunity sampling consists of selecting anyone who is available and willing to take part in the study at the time. This is a technique which is often used in psychological research due to its ease of application.

 

For example, an opportunity sample from a school that has 500 A‐level students in total would involve approaching the students who were, for example, in the sixth form centre during their free period to ask them to participate in a study. The first 20 who agree to take part would form part of the sample.

Evaluation of opportunity sampling

A strength of opportunity sampling is the convenient nature of the technique. In comparison to all other sampling methods, obtaining an opportunity sample is quicker and easier since it requires less effort on behalf of the researcher. As a result, it is likely to save money and is therefore favoured as the most economical technique.

 

There are issues of bias, however, with an opportunity sample. As the sample is drawn from a very specific area or location, e.g. university, this means that it is likely only students will be available to take part who are not representative of the target population. In addition, there in an increased risk of investigator bias as the researcher has complete control over who they approach. This means that they may select particular individuals or avoid others according to their own subjective preferences.

Volunteer sampling

Volunteer sampling consists of participants self‐selecting to take part in a study by either volunteering when asked or by responding to an advert.

 

For example, a psychologist could place posters in various locations around a school asking for A‐level students to volunteer to take part in their study, providing an email address to reply to or a time, date and venue to attend for participation. The first 20 volunteers would form part of their sample.

Evaluation of volunteer sampling

There are strengths of choosing to use a volunteer sample in a psychological investigation. In this way, participants generally approach the researcher rather than the other way around. This means that the technique requires minimal effort and input on behalf of the researcher. As a result, this makes obtaining a sample quicker and easier, in comparison to other methods.

 

There are issues of bias associated with volunteer sampling. Very often it is a particular type of person that is likely to take part in research as only those who see the advert will come forward to participate. Furthermore, those individuals who are more curious or inquisitive by nature may volunteer more readily. Therefore, the sample is likely to be biased and not representative of the target population which makes generalisation of the findings more difficult.

Comparison of sampling techniques

Possible exam questions

A A psychologist places an advert in a local newspaper, asking for participants.

B A psychologist uses lists of undergraduates from the nearby university and selects every fifth student to take part.

C A psychologist asks some of her mathematics students to take part in the research.

D A psychologist gives a number to all students in the psychology class then selects participants in an unbiased way.

After twenty participants completed the study, ten in each condition, the criminal psychologist compared the average number of items recalled in the cognitive interview with the average number of items correctly recalled in the standard police interview.

Name the sampling technique used in this experiment. (1 mark)

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