Sampling

• Population and Samples

  • Population is  the large group of individuals that a researcher is  interested in studying.
  • This is often called the target population
  • It is usually not possible to include all members of a target population in an investigation so a researcher selects a smaller group, known as the sample.
  • The sample needs to be representative of the target population so that generalisation can be made.
  • Samples are selected using a sampling techniques
• Random Sample

  • A random sample is a sophisticated form of sampling in which all members of the target population have an equal chance of being selected.
  • To select a random sample

  1. A complete list of the target population is obtained. 
  2. All names in the list are assigned a number.
  3. The sample is generated through the use of some lottery method 

  (ie. a computer-based randomiser or 

  picking numbers from a hat).
• Random Sample - AO3

  • Free from researcher bias
  • The researcher has no influence over who is selected and this prevents them from choosing people who they think may support their hypothesis.
• Random Sample - AO3

  • Difficult and time-consuming to conduct.
  •  A complete list of the target population may be extremely difficult to obtain.
  • You  may end up with a sample that is unrepresentative 
  • Probability suggest that random sampling is likely to produce a more representative sample than, say, opportunity sampling. However, it is still possible that the random method may not represent the sample.
  • Selected participants may refuse to take part (which means you end up with something more like a volunteer sample. This particular issue applies to all of the methods below.
• Systematic Sample

  • A systematic sample is when every nth member of the target population is selected, for example every 3rd house on a street or every 5th pupil on a school register.
  • A sampling frame is produced -  a list of people in the target population organised into - for instance, alphabetical order. 
  • A sampling system is nominated (every 3rd, 6th or 8th person, etc.) or this interval may be determined randomly to reduce bias.
  • The researcher then works through the sampling frame until the sample is complete.
• Systematic Sample - A03

  • This sampling method avoids researcher bias. 
  • Once the system for selection has been established the researcher has no influence over who is chosen (this is even more the case if the system is randomly selected).
  • It is also usually fairly representative. 

  For example, it would be possible, but quite unlucky, to get an all-male sample through systematic sampling.
• Stratified Sample

  • A stratified sample is when  the composition of the sample reflects the proportions of people in certain sub-groups (strata) within the population

  1. First the researcher identifies the different strata that make up the population. 
  2. Then, the proportions needed for the sample to be representative are worked out. 
  3. Finally, the participants that make up each stratum are selected using random sampling.
• Stratified Sample - AO3

  • Stratified sampling avoids researcher bias.
  • Once the target population has been sub-divided into strata, the participants that make up the numbers are randomly selected and beyond the influence of the researcher.
  • This method produces a representative sample because it is designed to accurately reflect the composition of the population. This means that generalisation of findings becomes possible.
• Stratified Sample - AO3

  • Stratified data cannot represent all the ways in which people are different and therefore can provide a complete representation of the target population
• Opportunity Sample

  • Given that representative samples of the target population are so difficult to obtain, many researchers simply decide to select anyone who happens to be willing and available (an opportunity sample). 
  • The researcher simply takes the chance to ask whoever is around at the time of their study, for example in the street (as in the case of market research).
• Opportunity Sample - AO3

  • This method saves a researcher a good deal of time and effort and is much less costly in terms of time and money than, say, random sampling.
• Opportunity Sample - AO3

  • Opportunity samples suffer from two forms of bias. 

  1. The  sample is unrepresentative of the target population as it is drawn from a very specific area, so findings cannot be generalised.
  2. Second, the researcher has complete control over the selection of participants (researcher bias).
• Volunteer Sample

  • A volunteer sample involves participants selecting themselves to be part of the sample
  • To select a volunteer sample a researcher may place an advert in a newspaper or on a common room notice board.
• Volunteer Sample - AO3

  • Collecting a volunteer sample is easy. It requires minimal input from the researcher ('they come to you') and so is less time-consuming than other forms of sampling.
• Volunteer Sample - AO3

  • Volunteer bias is a problem. 
  • Asking for volunteers may attract a certain 'profile of person’, that is, one who is helpful, keen and curious - which might then affect how far findings can be generalised.