Sampling

• The population


  Refers to a large group of individuals that a particular researcher may be interested in. E.g: children under 6 with autism. They are called the target population -subset of general pop
• Sample

  For practical/economic reasons, all of target pop can't be picked so a sample is chosen. Sample will be representative of target pop so that findings can be generalised
• Sampling techniques

  Due to the diverse nature of ppts in pop, there is a bias and thus,sampling techniques are used
• Random sample

  All members of the population have equal chance of being chosen. Firstly, complete a list of all members of target pop obtained. Secondly, number all the names and then sample is generated via use of lottery method
• Strengths of random sample

  Free from researcher bias as researcher has no influence over who is selected
• Limitations of random sample

  Difficult and time-consuming to conduct as a list of target pop may be difficult to obtain. Researcher may still end up with sample that is unrepresentative and selected ppts may refuse to take part
• Systematic sample


  Every nth member of target pop is picked. Sampling frame is produced- a list of people in target pop organised into alphabetical order for example. A sampling system is enforced or this interval may be determined randomly to deter bias. Researcher works through sampling frame until sample is complete
• Strengths of systematic sample

  Avoids researcher bias as once system for selection has been established, the researcher has no influence over who's chosen. Usually, the sample is representative
• Stratified sample


  Composition of sample reflects the proportions of people in certain subgroups (strata) withing target pop. First, researcher identifies the different strata making up pop. Then, the proportions needed for sample to be representative are worked out and the ppts that make up each stratum are selected using random sampling
• strengths of stratified samples

  Avoids researcher bias and the sample is representative because it is designed to accurately reflect the composition of the pop so generalisation is possible
• Limitations of stratified sample

  Complete representation of target pop is not possible as identified strata can not reflect all the ways people are different. Very time-consuming to identify subgroups, then randomly select participants and contact them.
• Opportunity sampling


  Many researchers simply decide to select anyone who is available at the time of exp
• Strengths of opportunity sampling

  Convenient as it saves researcher a good deal of time and effort, it is much less costly in terms of time and money
• Limitations of opportunity sampling

  Sample is unrepresentative of target pop as it is drawn from a very specific area. Researcher bias as researcher has complete control over selection of ppts and they might ignore they do not like the look of.
• Volunteer sample

  Ppts select themselves to be part of exp- self selection. Researcher may use posters/adverts or ppts could just simply raise their hands when researcher asks
• Strengths of volunteer sample

  Collecting a volunteer sample is easy and requires minimal input so it is less time-consuming
• Limitations of volunteer sample

  Volunteer bias is a problem as asking for volunteers may attract a certain 'profile' of person who could be helpful, keen and curious- can affect generalisation