PSYCH IB- Sampling

• opportunity sampling


  Recruit people who are most convenient or most available, for example people walking by you in the street or students at your school
• opportunity sampling-advantage


  the easiest method because you just use the first suitable participants you can find, which means it takes less time to locate your sample.
• opportunity sampling-disadvantage


  inevitably biased because the sample is drawn from a small part of the target population, for example if you selected your sample from people walking around the centre of a town on a monday morning it would be unlikely to include professional people who would be at work, or people from rural areas
• random sampling


  each item in a target population has an equal chance of being selected: the lottery method, or the random number generator
• random sampling-advantage


  unbiased; all members of the target population have an equal chance of selection
• random sampling-disadvantage 


  need to have a list of all members of the target population and then contact all of those selected, which may take some time
• snowball sampling 


  current participants recruit further participants from among people they know. thus the sample group appears to grow like a snowball
• snowball sampling-advantage 


  enables a researcher to locate groups of people who are difficult to access, such as drug addicts
• snowball sampling-disadvantage 


  the sample is not likely to be a good cross-section from the population because its friends of friends
• self-selected sampling


  advertise in a newspaper or on a notice board or on the internet
• self-selected sampling-advantage 


  gives access to a variety of participants which may make the sample more representative and less biased
• self-selected sampling-disadvantage 


  sample is biased in other ways because participants are likely to be more highly motivated to be helpful, and/or with extra time on their hands, this results in volunteer biased
• stratified and quota sampling


  subgroups (or strata) within a target population are identified (e.g. boys and girls, or age groups: 10-12 years, 13-15, etc.) participants are obtained from each of the strata in proportion to their occurrence in the target population. a stratified sample is done using a random technique. a quota sample is done using a non random technique
• stratified and quota sampling-advantage

  likely to be more representative than other methods because there is a proportional representation of subgroups
• stratified and quota sampling-disadvantage 


  very time-consuming to identify subgroups and then select participants and contact them
• systematic sampling


  use a predetermined system to select participants, such as selecting every 6th, 14th, 20th (etc) person from a phone book. the numerical interval is applied consistently
• systematic sampling-advantage


  unbiased as participants are selected using an objective system
• systematic sampling-disadvantage


  not truly unbiased/random unless you select a number using a random method and start with this person, and then select every nth person
• bias


  a systematic distortion
• sampling


  the selection of participants from the sampling frame with the aim of producing a representative selection of people from that group
• sampling frame


  the source material from which a sample is drawn
• target population


  the group of people that the researcher is interested in. the group of people from whom a sample is drawn. the group of people about whom generalisactions can be made.