Sampling Methods: Target Populations & Samples (OCR GCSE Psychology): Revision Note

Samples & target populations

Target population

• The target population is the entire group a psychologist wants to study

• This could be broad (e.g. all teenagers in the UK) or specific (e.g. teenagers with social anxiety)

• Researchers select a sample from this population to represent it in their study

Sample

• A sample is the smaller group of participants actually taking part in the research

• The aim is to make the sample representative, meaning it accurately reflects the target population in terms of characteristics such as:

  • age

  • gender

  • ethnicity

  • background

• When a sample is representative, results can be generalised to the wider population

Sample size

• Larger samples are generally more representative and reliable, as they reduce the impact of individual differences

• Small or specific samples make it difficult to generalise results beyond that group

  • E.g. studying 25 office workers from Glasgow may not represent office workers across the UK

• Larger samples also provide more robust quantitative data, making statistical analysis more accurate

• However, smaller samples may sometimes be necessary due to time, cost, or limited access to participants

Principles of sampling in scientific research

• Sampling should aim to:

  • reduce bias – everyone in the target population should have an equal chance of being included

  • increase representativeness – so findings can be generalised to the wider population

  • follow ethical guidelines – participants must give informed consent and take part voluntarily

• The quality of a sample directly affects how valid, reliable, and generalisable the findings are

  • Small or specific samples limit generalisation, while larger, more diverse samples produce more consistent and scientifically robust data

  • Cross-sectional samples, drawn from different locations or groups, are more representative than samples taken from one narrow group

  • Biased samples (e.g. all participants sharing the same age, town, or traits) reduce accuracy and may lead to misleading conclusions

• Researchers should aim for the most representative sample possible, but practical factors such as cost, time, and availability often restrict this

  • When such limitations exist, they must be acknowledged when interpreting results

Sampling methods

• Sampling methods are how a researcher obtains their sample

• Three key sampling methods are:

  • random sampling

  • opportunity sampling

  • self-selected sampling

Random sampling

• This is where every member of the target population has an equal chance of being selected

• Random sampling is usually done by using a computer-generated list, random number tables, or drawing names from a hat

  • E.g., selecting 100 students at random from a college of 1,000 to take part in a memory test

Strengths

• Likely to produce a representative sample with minimal bias compared to opportunity and self-selected samples

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

Weaknesses

• Can be time-consuming and difficult to select a sample if the target population is large

• Still possible to get an unrepresentative sample by chance

  • E.g. mostly males could be randomly selected

Opportunity sampling

• Participants are selected based on who is most available and willing at the time the research is being carried out

  • E.g. approaching people in a shopping centre to complete a quick questionnaire

Strengths

• Quick and convenient to carry out compared to random sampling

• Useful when the researcher has limited time or resources

Weaknesses

• Opportunity sampling produces biased samples, as people available at one time or place may share similar traits (e.g. students in a university library)

• This means that findings may lack generalisability

Self-selected sampling

• Participants volunteer to take part, often by responding to an advert online, in a newspaper, on social media or on a notice board

  • E.g. posting an advert on a community Facebook page asking volunteers to take part in a sleep study

Strengths

• Participants are usually motivated and have given informed consent, as they come forward willingly to participate in the research

• Can reach a wide audience through online recruitment

Weaknesses

• Can lead to volunteer bias — certain types of people (e.g. confident, curious, or with more free time) are over-represented in research

• This means that the sample may be unrepresentative of the wider population