Research methods - Sampling

• Groups of ppts that form the research process are selected through what process?

  Sampling
• Define population


  Group of people who the researcher is interested in studying
• What is the researcher’s population often called?

  Target population - a subset of the general population
• Define sample


  A group of people who take part in an investigation - drawn from the target population and presumed to be representative
• Why should the sample drawn from the target population be representative?

  To be able to generalise findings
• How are samples selected?

  Via sampling techniques
• What are the 5 types of sampling techniques?

  • Random sample
  • Systematic sample
  • Stratified sample
  • Opportunity sample
  • Volunteer sample
• What is a random sample?

  • All members of target population have equal chance of being selected
  • Need to obtain complete list of all members of TP - then all names are assigned a number
  • Sample selected through lottery method to ensure it is random
• What is a systematic sample?

  • When every Nth member of the TP is selected e.g. every 5th pupil on a school register
  • Sampling frame is produced - a list of people in TP organised into an order e.g. alphabetical
  • Sampling system is nominated (e.g. every 5th)
  • Can begin from a randomly determined start to reduce bias
  • Researcher works through sampling frame until sample is selected
• What is a stratified sample?

  • The composition of sample reflects the proportions of certain subgroups within the TP via strata
  • Researcher identifies the different strata that make up the population
  • Work out proportions needed for sample to be representative
  • Ppts that make up each stratum are selected using random sampling
• What is an opportunity sample?

  • Researchers select anyone who is willing & available
  • Takes chance to ask whoever is around at time of study
• What is a volunteer sample?

  • Ppts self select themselves to be part of the sample (self selection)
  • Researcher might place advert
  • Those who self select themselves would then make up the sample
• What are the evaluation points for random sampling technique?


  • Potentially unbiased - confounding variables equally divided between the groups enhancing internal validity
  • Difficult & time consuming to conduct - list of TP may be difficult to obtain
  • Can still end up with a unrepresentative sample - never certain
• What are the evaluation points for systematic sampling technique?

  • Objective method - little room for bias : researcher has no influence over who’s chosen once system for selection has been established
  • Method is time consuming & ppts can refuse to take part
• What are the evaluation points for stratified sampling technique?

  • Can generalise findings as it produces a representative sample - designed to reflect the composition of the population
  • Identified strata might not always reflect the TP accurately - there isn’t complete representation
• What are the evaluation points for opportunity sampling technique?

  • Convenient & less costly e.g. no list required
  • Suffer from 2 types of bias : sample is unrepresentative as it’s drawn from specific area (can’t be generalised to TP) & researcher has complete control over selection of ppts (researcher bias)
• What are the evaluation points for volunteer sampling technique?

  • Convenient & less time consuming - requires minimal input from researcher
  • Volunteer bias - those who volunteer may have a certain ‘profile’ e.g. curious/pleasing. This can then affect how far the findings can be generalised