🏫 Population
All the pupils at Eastfield School. They have one thing in common: they attend the same school.
Sampling » Populations, Samples and Sampling Techniques
What you'll learn this session
Study time: 30 minutes
Cambridge spec: 7.5.1, 7.5.2, 7.5.3, 7.5.4
- What a population and a sample are
- What a sampling technique is
- How opportunity sampling and volunteer sampling work
- The strengths and weaknesses of each, and how to apply them to a new scenario
Sampling questions appear in Section D of both papers, so it is worth getting them right. Every piece of research needs people to take part. This lesson is about who those people are and how researchers choose them.
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Unlock This CoursePopulation and sample
A researcher cannot test everybody. Imagine you want to know how much sleep teenagers get. You could never ask every teenager in the world. Instead, you choose a smaller group and study them.
Key terms:
- Population: a group of people with one or more characteristics in common, from which a sample is selected.
- Sample: a group of people selected to represent the population in research.
The characteristic in common can be almost anything: being aged 14 to 16, being left-handed, or being a pupil at one school. The population is always the big group. The sample is the small group taken from it.
👥 Sample
30 of those pupils who actually take part in the study. They stand in for everyone at the school.
Learn this
The sample is meant to represent the population. The population is the group the researcher wants to say something about. The sample is the group who really take part.
Sampling techniques
Once the researcher knows the population, they need a way to pick the sample. For this course you need two techniques: opportunity sampling and volunteer sampling.
- Sampling technique: the method used to select research participants from the population.
Opportunity sampling
An opportunity sample is made up of participants who are selected because they are available at the time of the research. The researcher simply asks whoever happens to be there.
For example, a researcher stands outside a library on Saturday morning and asks the first 20 people who walk out to complete a short questionnaire. The researcher chose them because they were there at the time.
✅ Strengths
It is quick and easy. The researcher does not need a list of the population or any invitations, so the sample can be collected in a short time.
❌ Weaknesses
The sample may not represent the population. Only people who happen to be there at that time can be chosen, so some groups may be missing. The library visitors on a Saturday may not be like the rest of the town.
Volunteer sampling
A volunteer sample is made up of participants who choose to take part in research by responding to an invitation to participate. The researcher does not pick them. They put themselves forward.
For example, a researcher puts up a poster in a sports centre and posts a notice online asking for people to help with a study on exercise habits. Everyone who replies and agrees to take part becomes the sample.
✅ Strengths
The participants have chosen to take part, so they are usually willing to finish the study. The researcher can reach a wide range of people through an advert, not just those nearby.
❌ Weaknesses
The sample may not represent the population. People who volunteer may be keener or more interested than others. For an exercise study, mostly sporty people might reply.
Opportunity or volunteer?
Opportunity: the researcher approaches people who are available. Volunteer: people respond to an invitation. Ask yourself: did the researcher go to them, or did they come forward?
Random allocation, which you met earlier, is not sampling. It decides which condition a participant takes part in once they are already in the study. Sampling is about how the participants are selected in the first place.
Applying it to a new scenario
Worked example
A researcher wants to find out how many hours of homework pupils at a large secondary school do each week. A notice is read out in assembly asking any pupils who would like to help to give their names to the researcher. Twenty-five pupils do so.
Population: all the pupils at the secondary school.
Sample: the 25 pupils who gave their names.
Sampling technique: volunteer sampling, because the pupils responded to an invitation.
Strength: the pupils chose to take part, so they are likely to complete the study. Weakness: pupils who volunteer may be more hard-working than others, so the sample may not represent all pupils.
The same school could be studied with an opportunity sample instead. The researcher could go to the canteen at lunchtime and ask the first 25 pupils they see. The pupils did not volunteer. They were simply available at the time.
How well a sample represents the population links to validity, which you will meet in the next lesson.
Common mistakes
- Mixing up population and sample. The population is the whole group. The sample is the smaller group taken from it.
- Calling a volunteer sample "random". Volunteers choose themselves. No random selection is involved.
- Writing "it is biased" with no reason. Say the sample may not represent the population, and why.
- Writing "it is cheap" as the only strength of opportunity sampling. Say it is quick and easy because the researcher uses people who are available.
Exam-style question
A psychologist wants to study how much time teenagers in a town spend on social media. The psychologist stands outside the town's youth centre one evening and asks the first 15 teenagers who arrive to complete a questionnaire.
(a) Identify the population in this study. [1 mark]
(b) Name the sampling technique used. [1 mark]
(c) Explain one weakness of using this sampling technique in this study. [2 marks]
Model answer
(a) Teenagers in the town (1).
(b) Opportunity sampling (1).
(c) The sample may not represent the population (1) because only teenagers who went to the youth centre that evening could be chosen, and they may use social media differently from teenagers who stayed at home (1).
Exam tip
For a weakness question, use the details in the scenario. Say who was left out or who might be different, such as teenagers who did not go to the youth centre.