Populations and samples

• sampling methods

  random

  systematic

  stratified

  opportunity

  volunteer
• Random

  = obtain all names of target population, assign a number to each name, pick number from computer.

  • considered representative as every person has same chance of being picked.
  • free from researcher bias, can generalise to target population
  • can be time consuming
• Systematic

  = every 3rd person on list is chosen, the interval is determined randomly to reduce bias.

  + avoids researcher bias, no influence over who is chosen, representative.

  -can be unlucky and get a lack of representation
• Stratified

  =sample that reflects all the sub-groups within target population.- first work out all possible sub-groups eg: gender, age. Then randomly select a proportion of each stratum.

  + avoids researcher bias, representative sample.
• opportunity

  = researcher will select anyone who is willing and available at the time.

  + convenient, saves time, effort, money.

  -open to bias: unrepresentative, researcher has complete control over selection and can be biased in choosing participants.
• volunteer


  = self-selection, people volunteer to take part.

  + easy to obtain, less time consuming

  -asking for volunteers can produce biased sample, usually selects a certain 'profile' of person. eg: helpful, keen, curious.