Sampling techniques

• Population

  The group of people the researcher is interested in (aka the target population).
• Sample

  Smaller group selected from the target population to carry research out on.
• Sampling

  Method used to get a smaller group of potential research participants from the population.
• Opportunity sample

  Selecting those most easily available at the time of the study.
• Opportunity sample: How?

  Stopping people who are nearby, walking past and asking them to participate.
• Opportunity sample: Strengths

  Quick and easy- takes less time than other methods.

  Inexpensive
• Opportunity sample: Weaknesses

  Bias: Sample drawn from the small part of the population, therefore lacks population validity.

  Issues with generalisability: may be misrepresentative.
• Random sample

  Every member of the target population has an equal chance of being selected.
• Random sample: How?

  School registers, census, electoral/voting lists, assigning a number to each member then selecting via random number generator.
• Random sample: Strengths

  Unbiased: all have an equal chance

  Widely accepted

  Reasonable chance of representative sample.
• Random sample: Weaknesses

  Time consuming: need to have a list of all of the members of the target population, then contact them, it could be time consuming.

  Can be impractical.
• Stratified sample

  Sub-groups are identified and participants are selected from these sub-groups. They must be proportional and reflect the target population.
• Stratified sample: How?

  Percentage of individual groups in the study match the percentage of the wider population.

  (Number in group ÷ total) * sample size required
• Stratified sample: Strengths

  Proportional and random selection of each sub-group, more representative.
• Stratified sample: Weaknesses

  VERY time consuming- subgroups must be identified, ppts randomly selected and then contacted.

  Care must be taken to ensure each key characteristic is selected across strata.
• Systematic sampling

  Selecting every nth person.
• Systematic sample: How?

  Select every 'k' person from potential participants. E.g:

  Every 10th person who walks by

  Every 3rd address

  K = N(total target population)/n(sample)
• Systematic sample: Strengths


  Unbiased: objective system
• Systematic sample: Weakness

  Not truly unbiased unless you randomly select a number in the first place and then start with this person.
• Volunteer sampling


  Relies solely on people volunteering to take part
• Volunteer sampling: How?


  Advert, questionnaire, asking people if they want to participate.
• Volunteer sampling: Strengths

  Good to use with a specialised/ sensitive topic.

  Less effort than others.

  Lower drop out rate because ppts want to be there.

  Large sample size.
• Volunteer sampling: Weakness

  Bias

  Generalisability: volunteers are likely to be more enthusiastic in the study which makes it less generalisable. All volunteers may display similar characteristics as they all volunteered (e.g. extroverted, trusting, cooperative).