Sampling

• what is a population?

  a group of people the researcher who are the focus of a study
• what is a sample ?

  a small group of people drawn from the target population and is presumed/ aimed to be the representative of that pop
• why do investigators use sampling?

  not possible to include all members of a target pop due to practical and economic reasonings
• why should the sample be representative?

  so generalisations of the study is possible
• what is random sampling?

  sophisticated form of sampling where all members of target pop have an equal chance of being selected

  1. complete list of all members of target pop
  2. all members are assigned a number
  3. sample selected through some use of lottery method e.g random number generator
• what is a strength of random sampling?

  no researcher bias (investigator affects as they have no influence of who is picked = removal of CVs and EVs as they are all equally divided between diff groups = enhances internal validity
• what are the limitations of random sampling?

  time consuming (need to get list of all members of pop) + still may be unrepresentative as some pps may not choose to participate which may result in volunteering sampling
• what is systematic sampling?

  A predetermined system is used whereby every nth member is selected from the sampling frame.

  • researcher works through the sampling frame until sample is complete
• what is a strength of systematic sampling?

  it is objective so once system for selection has been established the researcher has no influence over who is chosen which avoids researcher bias
• what is a limitation for systematic sampling?


  some pps may not choose to participate
• what is stratified sampling?

  the composition of the sample reflects the varying proportions of people in particular subgroups (strata) within the wider population.

  1. you identify diff strata that make up the pop
  2. calculate the required proportion needed for each stratum based on the target pop
  3. select sample at random from each stratum using a random sampling
• what is a strength of stratified sampling?

  Produces representative data due to the proportional strata designed to reflect the composition of pop hence generalisation is possible.
• what is a limitation of stratified sampling?

  Time consuming to identify strata and contact people from each + complete representation of the target pop not possible as the identified strata cannot reflect all the diff between the people of the wider pop
• what is volunteer sampling?

  Involves self selection whereby the participant offers to take part either in response to an advert or when asked to
• what is a strength of volunteer sampling?

  Quick access to willing pps = easy and not time consuming unlike the other methods of sampling + as pps are willing to take part they are more likely to cooperate + engage in the study.
• what is a limitation of volunteer sampling?

  Volunteer bias study may attract a particular profile of a person = generalisability is then affected + motivations like money could be driving participation so pps may not take study seriously, influencing the results or respond to demand characteristics to get money
• what is opportunity sampling?

  Pps happen to be available at the time which the study is being carried out so are recruited conveniently as representative samples of pop may be diff to obtain
• what is a strength of opportunity sampling?

  Easy method of recruitment which is time saving and less costly.
• what is a limitation of volunteer sampling?

  Not representative of the whole pop hence lacks generalisability + researcher bias is presented as they control who they want to select.