Sampling methods

• Population


  Group of people who are the focus of the researchers interest from which a smaller sample is drawn from.
• Target population


  Population that a study is intended to research and to which generalise from samples are to be made.
• Sample


  A group of people who take part in a research investigation. Represents the population. It is taken from the target population.
• Participant


  Individual who participates in an investigation, analysis of an experiment.
• Opportunity Sampling method


  Simply selecting those people that are available at the time.

  For example, going up to peoples cafes or on the street and asking them to be interviewed.
• Opportunity Sampling method advantages


  Quick, convenient and economical

  The most common type of sampling in practice.
• Opportunity Sampling method disadvantage

  Very unrepresentative/biased samples, which are unusually small, often biased by the researcher who will likely choose people who are 'helpful'. So not generalisable.
• Random Sampling method


  Every member of a target population has an equal chance of being selected.

  For example, pulling names out of a hat or random number generator
• Random Sampling method advantages


  For very large samples it provides the best chance of an unbiased representative sample, as everyone has an equal chance
• Random Sampling method disadvantages

  - For large populations it is time-consuming or impossible to create list of every individual from a target population

  - Can still get a biased sample, i.e all males
• Stratified Sampling method


  The composition of the sample reflects the proportions of people in certain sub-groups (strata) within the target population.
• Stratified Sampling method advantage

  - Avoids researcher bias

  - Representative sample obtained which mirrors the target population directly, so generalisation of findings is possible
• Stratified Sampling method disadvantage

  The identified strata cannot reflect ALL the ways that people are different, so complete representation of the target population is not possible
• Systematic Sampling method

  Every nth member of the target population is selected
• Systematic Sampling method advantages

  - Avoids bias

  - Is fairly representative i.e. it would be very unlikely to get al all-male sample selecting people from a list in this way
• Systematic Sampling method disadvantages

  Not strictly random as each person does not stand an equal chance of being selected from the full target population list.
• Volunteer Sampling method


  Individuals who have chosen to be involved in a study, also called slef-selecting.
• Volunteer Sampling method advantages

  - Relatively convenient, quick and ethical if it leads to informed consent

  - can be targeted specifically at certain target population target population
• Volunteer Sampling method disadvantages


  - Unrepresentative as it leads to bias on the part of the participant.

  - Only people who see the Ad have a chance to be in the study

  - Participants are more motivated, are not typical (=volunteer bias), so not generalisable