Selection of Participants

• Methods for selection of participants?

  1. Random sampling
  2. Stratified sampling
  3. Convenience sampling
  4. Snowball sampling
  5. Random allocation
• Define population:

  Total group of individuals from which sample can be drawn
• Define sample:

  A representative group of the population
• Describe conveinience sampling:

  Uses participants who are willing and available at the time
• Strengths of convineince sampling:

  • Time and effort to collect sample is low compared to random and stratified sampling.
  • Costs to gather sample is low.
• Limitations of convineince sampling:

  • May be high level of researcher bias
  • May not be representative
• Describe the snowball sampling method:

  • Initial participants are chosen then:
  • Each participant encourages other people to contact the researcher and join the sample
• Strengths of Snowball sampling:

  • Researcher able to find a sample that may be difficult to recruit due to the nature of the study
  • Example: Drug users
  • Less time needed
• Limitations of snowball sampling:

  Unlikely to be representative of the population it was taken from

  May be biased as only those who are in direct contact with the original participants can be recruited
• Describe random sampling:

  • Everyone in the population has an equal chance of being selected
  • Requires identifying the target population and using raffle method
• Strength of random sampling:

  • Quick and inexpensive
  • Likely to be more representative than convenience sampling
• Limitation of random sampling:

  • Sample may not be representative of the population (may be biased)
• Describe stratified sampling:

  • Researcher identifies different demographic groups (strata) within the population
  • Then works out the proportions required for each characteristic.
  • Once population is divided into strata, participants are selected from the strata.
• Strength of stratified sampling:

  • Highly representative of the target population
• Limitation of stratified sampling:

  • Time consuming because of the need to obtain information about population characteristics
• Describe random allocation:

  • Random distribution of participants into experimental and control groups
• What does random allocation reduce and increase?

  • Reduces researcher bias
  • Increases generalizability of results.
• Method for random allocation

  • Names of participants in the sample are collated.
  • Names are randomly selected by drawing them out of a hat or using a computer-generated program and assigned to groups.
• Strengths of random allocation:

  • Good generalisability of results because equivalent groups of participants have been created.
  • Prevents selection bias because each participant has an equal chance of being placed in different conditions.
• Limitations of random allocation:

  • Can't use when the independent variable is not able to be manipulated by the researcher.