Sampling methods

What is a population?

The whole group of people that a particular researcher is interested in studying.

• Example: All teenagers in the UK

What is a target population?

A more specific group the researcher intends to study and from which they want to draw conclusions because they’re practical to reach and fit the research aims.

• Example: Teenagers in the UK aged 13–15 who attend school

What is a sample?

A group of people who take part in a research investigation. It’s drawn from a (target) population and is presumed to be representative of that population.

• i.e. It stands fairly for the population being studied

Why do researchers study a target population instead of the whole population?

Because studying the whole population is too big and unrealistic.

• A target population is a smaller, specific group that is easier to access, cheaper to study, and keeps the research more controlled

If a sample is truly representative, what should psychologists be able to do?

Generalise the conclusions of the study to the whole target population.

What is a biased sample?

A sample where certain people are more likely to be chosen than others, so the sample ends up unbalanced and not like the wider group.

• A researcher wants to study all teenagers, but they only ask teenagers from one private school, the sample is biased — it doesn’t reflect all teens

What is a representative sample?

A sample where the people in it have the same key characteristics as the target population (e.g., similar ages, genders, backgrounds).

• It means the sample looks like the group the researcher wants to draw conclusions about

What is generalisability?

The extent to which research findings can be applied to a population from the sample.

What is stratified sampling?

A sophisticated form of sampling in which the composition of the sample reflects the varying proportions of people in certain subgroups (strata) within the target population or the wider population.

How do you carry out stratified sampling?

1. The researcher identifies the different strata that make up this population

2. The proportions needed for the sample to be representative are worked out

3. The participants that make up each stratum are selected using random sampling

What are the strengths of stratified sampling?

• The sample is representative of the target population as it is based on exact proportions of the target population

  • This means that it is easy to generalise data from the sample to the wider population (with more confidence)

• The researcher has control over the chosen categories, which can be selected according to how relevant they are in terms of the research aim

What are the limitations of stratified sampling?

• It can be difficult when researchers cannot confidently classify every member of the population into a subgroup

• Gathering the sample population can be time-consuming

  • It is not always possible to get access to all the information on a target population

What is systematic sampling?

When every nth member of the target population is selected from a list to make a sample.

• For example: Every 3rd house on a street or every 5th pupil on a school register

How do you carry out systematic sampling?

1. A sampling frame is produced, which is a list of people in the target population organised into, for instance, alphabetical order

2. A sampling system is nominated (every 3rd, 6th or 8th person etc.) or this interval may be determined randomly to reduce bias

3. The researcher then works through the sampling frame until the sample is complete

What are the strengths of systematic sampling?

• Its’s an unbiased sampling method, as the researcher has no control over where participants are placed on the population list - avoids researcher bias

  • This means that the sample is more fairly representative than is obtained by some other sampling methods

  • A more representative sample can be generalised more easily

• Systematic sampling is a quick, easy and cost-effective method

  • This makes it popular amongst researchers and likely to be agreed by funding bodies

What are the limitations of systematic sampling?

• This method is not completely free from bias as the selection process can interact with a hidden periodic trait (when every nth person in the sample has the same or similar characteristics)

  • If every 10th person on the list is a 19-year-old female nurse, then this would constitute the sole demographic in the sample

• A researcher using systematic sampling has to know the size of the population to generate the optimum sample size

  • Without this information the sample may lack generalisability

What is random sampling?

A sophisticated form of sampling in which all members of the target population have an equal chance of being selected.

How do you carry out random sampling?

1. A complete list of all members of the target population is obtained

2. All of the names on the list are assigned a number

3. The sample is generated through the use of some lottery method (a computer based randomiser)

What are the strengths of random sampling?

• This sampling method eliminates researcher bias as the researcher has no control over who is selected to be in the sample

• Using a random sample means that the study results should be fairly representative

  • This means that the findings can be generalised to the target population

What are the limitations of random sampling?

• Can be time-consuming and impractical

  • It is not always possible to get access to all the information on a target population

  • Additionally, not everyone selected for the sample may want to take part in the study

• Random sampling can result in a unrepresentative sample

  • Due to the nature of the method, the sample could be unbalanced

    • An all-male sample could be selected randomly, which may not be a true representation of the target population 

What is volunteer sampling?

Involves participants selecting themselves to be part of the study hence, it’s also referred to as self-selection.

How do you carry out volunteer sampling?

The researcher may place an advert in a newspaper or on a common room notice board.

What are the strengths of volunteer sampling?

• Quick, easy and cost-effective

  • It is one of the most used (probably the most popular) sampling methods by psychologists

• Participants are likely to be willing and enthusiastic about the research

  • They have made a conscious decision to take part in the research, which means that they are less likely to jeopardise the study and its results

What are the limitations of volunteer sampling?

• This method is prone to volunteer bias

  • The results are not easy to generalise as volunteer participants often have personality traits in common, e.g. sociable, outgoing, etc.

• Volunteers are often eager to please the researcher

  • They’re more cooperative and motivated than the average person, making the sample biased

  • This tendency to please the researcher may give rise to demand characteristics, which in turn affects the validity of the findings

What is opportunity sampling?

When the researcher obtains their sample from those who are present and available at the time and who are willing to take part in the research. AKA convenience sampling.

• E.g. people who are shopping in a local supermarket at 11am

How do you carry out opportunity sampling?

The researcher simply takes the chance to ask whoever is around at the time of their study.

What are the strengths of opportunity sampling?

• The 'convenience' aspect of opportunity sampling is a strength, as it is a quick and easy way of obtaining participants for a study

• As people have been approached and agreed to take part, this is more likely to mean that the research process runs smoothly

  • Having unwilling or resistant participants could damage the validity of the findings

What are the limitations of opportunity sampling?

• It’s not possible to generalise from an opportunity sample, as the sample only represents those who were available and willing to participate at the time

• The researcher may be prone to unconscious bias when they approach people to take part in the study, e.g.,

  • they may tend to approach people they feel comfortable with

  • they may select only those participants who they think will be interested

  • they may avoid some social groups that they are wary of, e.g., males aged 18-25

How can random sampling also be carried out?

By using manual selection. The researcher puts each name (or assigned number) on a separate slip of paper and places them all in a container. The researcher then selects X amount of slips from the container (the number of people they want in their sample).

What rules should apply to manual selection?

• Shake the container between each draw

• The slips of paper should all be the same size and folded in the same way so that one doesn’t feel different from another

• The selector draws blind i.e. can’t see the actual slips of paper (blindfolded)