Sampling

• What is a population?

  A group of people who are the focus of the researcher's interest, from which a smaller sample is drawn.
• What is a sample?

  A group of people who take part in a research investigation. The sample is drawn from a (target) population and is presumed to be representative of that population i.e. it stands 'fairly' for the population being studied.
• What is a sample technique?

  The method used to select people from the population.
• What is bias (in context of sampling)?

  When certain groups are over/under-represented within the sample selected.

  --> this limits the extent to which generalisations can be made to the target population
• What is generalisation?

  The extent to which findings and conclusions from a particular investigation can be broadly applied to the population. This is possible if the sample of participants is representative of the target population.
• What is a 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.

  Steps in selecting a random sample:

  1. Obtain a complete list of all members of the target population
  2. All of the names on the list are assigned a number
  3. The actual sample is selected through the use of some lottery method (a computer/phone randomiser or picking numbers from a hat)
• What is a systematic sample?

  A systematic sample is when every nth member of the target population is selected e.g. every 3rd house on a street or every 5th pupil on a school register.

  • A sampling frame is produced, which is 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.)
  • May begin from a randomly determined start to reduce bias
  • The researcher then works through the sampling frame until the sample is complete
• What is a stratified sample?

  A stratified sample is a sophisticated form of sampling in which the composition of the sample reflects the proportions of people in certain subgroups (strata) within the target population or the wider population.

  How a stratified sample is carried out:

  1. The researcher identifies the different strata that make up the population
  2. The proportions needed for the sample to be representative are worked out
  3. The participants that make up each stratum are selected using random sampling
• What is an 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.

  • The researcher simply takes the chance to ask whoever is around at the time of their study e.g. in the street (in the case of market research)
• Evaluate the use of random sampling

  Strengths:

  • potentially unbiased - this means that confounding or extraneous variables should be equally divided between the different groups, enhancing internal validity

  Limitations:

  • difficult and time-consuming to conduct - a complete list of the target population may be extremely difficult to obtain
  • may end up with a sample that is still unrepresentative
  • selected participants may refuse to take part (which means you end up with something more like a volunteer sample)
• Evaluate the use of systematic sampling

  Strengths:

  • the sampling method is objective - once the system for selection has been established the researcher has no influence over who is chosen

  Limitations:

  • time-consuming and, in the end, participants may refuse to take part, resulting in a volunteer sample
• Evaluate the use of stratified sampling

  Strengths:

  • 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

  Limitations:

  • the identified strata cannnot reflect all the ways that people are different, so complete representation of the target population is not possible
• Evaluate the use of opportunity sampling

  Strengths:

  • convenient - method is much less costly in terms of time and money than e.g. random sampling, as a list of members of the target population is not required, and there is no need to divide the population into different strata unlike stratified sampling

  Limitations: (two forms of bias)

  1. sample is unrepresentative of the target population as it is drawn from a very specific area so findings cannot be generalised to the target population
  2. researcher has complete control over the selection of participants so there could be researcher bias
• Evaluate the use of volunteer sampling

  Strengths:

  • collecting a volunteer sample is easy - minimal input required from the researcher (they come to you) therefore is less time-consuming than other forms of sampling
  • researcher ends up with participants who are more engaged, more so than someone who was stopped in the street

  Limitations:

  • volunteer bias - asking for volunteers may attract a certain 'profile' of person, that is, one who is curious and more likely to try to please the researcher (which might then affect how far findings can be generalised)