Sampling Methods (AQA A Level Psychology): Revision Note

Population & sample

• At the beginning of the research process, the researcher must identify the target population, e.g.,

  • women aged 35-50 years old who have survived breast cancer

  • people who live in large cities in the UK

  • university students who are taking STEM subjects

• The sample used in psychological research is taken from a target population

• Often a sample is taken from a population which is more generalised than a target population

  • A researcher who wishes to investigate the effects of being a single teen parent will require their sample to be drawn from that specific population

    • This is an example of a distinct target population

  • A researcher is running an experiment on the duration of short-term memory (STM)

    • Generally, anyone from the age of 18 to 60 years old would suffice i.e., a distinct target population is not required

• The researcher draws the sample from the population to take part in the experiment and then generalises the findings across the target population

Random sampling

• Random sampling involves obtaining a sample taken from a population in way which has the least bias out of all of the sampling methods

• With random sampling, every member of the population has an equal chance of being selected

• How a random sample could be achieved includes:

  • putting all the names of the population in a hat and then drawing out one name at a time until the optimum sample size has been reached (e.g., a sample size of 50)

  • employing a computer name-generator software (this is more useful if a large sample is required, e.g., 2,000 participants)

Evaluation of random sampling

Strengths

• 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

Limitations

• 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 non-representative 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 

Systematic sampling

• Systematic sampling involves selecting every nth person from a list to make a sample, e.g.,

  • the researcher selects, for example, every 10th, 100th or 1000th on a register/database/roll depending on the size of both the population and the sample required

• The sampling interval (e.g. every 100th person) is calculated by dividing the population size by the required sample size e.g.,

  • from a population of 100,000, a sample of 2500 is required

  • 100,000 divided by 2500 = 40

  • thus, every 40th person on the population list is selected for the sample

Evaluation of systematic sampling

Strengths

• This is an unbiased sampling method, as the researcher has no control over where participants are placed on the population list

  • This means that the sample is more 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

Limitations

• This method is not completely free from bias as the selection process can interact with a hidden periodic trait

  • 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

Stratified sampling

• Stratified sampling generates a small-scale reproduction of the target population

  • The target population is divided and categorised according to key characteristics required by the research, e.g.,

    • age

    • gender

    • education level

    • ethnicity

    • profession

• The population is sampled within each category proportional to the overall total, e.g.,

  • If the whole population has a total of 18% of males aged 30-40, then the representative sample will have 18% of males aged 30-40

Evaluation of stratified sampling

Strengths

• 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

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

Limitations

• Stratified sampling 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

Opportunity sampling

• Opportunity sampling involves the researcher obtaining their sample from those who are present and available at the time and who are willing to take part in the research, e.g.,

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

  • university students who are present at one of the researcher’s lectures

  • young parents who are attending a baby yoga group

• Opportunity sampling is also known as convenience sampling

Evaluation of opportunity sampling

Strengths

• 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

Limitations

• It is 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

Volunteer sampling

• Volunteer sampling involves people actively selecting themselves to participate in a study, i.e., they volunteer to take part

  • Volunteer sampling is also known as self-selecting sampling

• A researcher finds a specific way or ways in which to find their sample, e.g.,

  • putting up posters and flyers around a university campus asking for volunteers to participate in a study

  • posting an advert on various social media platforms

  • running an advert in a print newspaper

• The research will advertise when and where the study is taking place and how they can participate

• The advert may specifically ask for people with certain characteristics, e.g.,

  • first-time parents

  • people with ADHD

  • bilingual people

Evaluation of volunteer sampling

Strengths

• This method is 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

Limitations

• 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

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