methods of sampling

• Sampling methods


  Different strategies researchers use to get participants for their studies
• Target population


  The group of individuals the researcher is interested in
• The aim of the researcher is to try and take a representative sample from the target population using a sampling method

  
• If the sample is representative, findings can be generalized to the target population

  
• Main sampling methods


  • Random sampling
  • Opportunity sampling
  • Systematic sampling
  • Stratified sampling
  • Volunteer sampling
• Random sampling


  1. Identify members of the target population

  2. Number them

  3. Attempt to draw out the required number of people for the study

  4. Selection of participants can be done in a randomised way

  
• Random sampling


  • All members of the target population have an equal chance of being selected
• Strengths of random sampling

  • Unbiased as all members of the target population have an equal chance of being selected
  • More representative of the target population
  • More generalisable
  • Less chance that the researcher can influence the results
• Weaknesses of random sampling

  • Possibility of an unbalanced and biased sample by chance, particularly if the sample size is too small
  • Time-consuming to gather randomised samples
  • Participants can refuse to participate
• Stratified sampling


  1. Identify sub-groups (or strata) within the population

  2. Gather participants from each strata in proportion to their occurrence in the population

  3. Selection of participants is generally done using a random technique

  
• Strengths of stratified sampling

  • Very representative of the population
• Weaknesses of stratified sampling

  • Can be time consuming and impractical
  • People can still refuse to take part, leading to sample error and invalid conclusions
• Volunteer sampling


  Participants who volunteer to take part in a study
• Strengths of volunteer sampling

  • Minimal effort on part of researcher
  • Most ethical as participants do not have to be asked
  • Volunteers are likely to be motivated and act/respond honestly
  • Fast and efficient
• Weaknesses of volunteer sampling

  • Less representative as not everyone in the target population will see the advert or want to respond
  • Volunteer bias as those who volunteer may be a particular type of person
  • Findings might be unreliable
• Opportunity sampling


  Asking those who are around you and most easily available, that represent the target population, to participate in the study
• Strengths of opportunity sampling

  • One of the fastest and easiest ways to gather participants
• Weaknesses of opportunity sampling

  • Greater chance of being biased as the sample is drawn from a very narrow part of the target population
  • Less generalisable
  • Participants may try to "help" the researcher in a way that would support the hypothesis, making the results unreliable and invalid
  • The process is not randomised, so the investigator may select participants they know will behave in a certain way
• Systematic sampling


  Selecting every "nth" member of the target population
• Strengths of systematic sampling

  • Unbiased and simple
• Weaknesses of systematic sampling

  • Participants gathered could still be unrepresentative and due to chance selection, making the results unreliable when re-tested
  • Need a bigger sample size to be able to filter out participants based on the "nth" selection