Sampling

• Population


  A group of people who are the focus of researchers interest from which a smaller sample is drawn.
• Sample


  A group of people who take part in the research investigation. Sample is drawn from target population and is presumed to be representative of that population.
• Sampling techniques

  Method used to select people from a population.
• Bias


  In the context of sampling, when certain groups are over or under represented within the sample selected.

  E.g too many younger people, too many people of one ethnic group.

  This limits the extent to which generalisation can be made to target population.
• 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.
• Random sample

  • A form of sampling where all members of the target population have an equal chance of being selected.
• How is a random sample carried out?

  1. Obtain a complete list of all members of the target population.
  2. All names on the list are assigned a number.
  3. Actual sample is selected through the use of a lottery method e.g random number generator.
• Strength of random sampling

  • Unbiased, this means confounding or extraneous variables should be equally divided between the groups, enhancing internal validity.
• Limitations of random sampling

  • Difficult and time consuming to conduct, a complete list of the target population may be hard to obtain.
  • Sample may still be unrepresentative.
  • Selected participants may refuse to take part.
• Systematic sample

  • When every nth number of the target population is selected.
  • E.g every 3rd house on a street.
  • A sampling frame is produced, which is a list of people in the target population organised. E.g alphabetical order.
  • May begin from a randomly determined start to reduce bias.
  • Researcher works through sampling frame until sample is complete.
• Strengths of systematic sample

  • Sampling method is objective, once the system for selection has been established the researcher has no influence over who is chosen.
• Limitation of systematic sampling

  • Time consuming and participants may refuse to take part, resulting in a volunteer sample.
• Stratified sampling

  • Sampling method where the composition of the sample reflects the proportions of people in different sub-groups within the target population.
• Explain how stratified sampling works

  1. Researcher identifies strata that makes up population.
  2. Proportions needed for the sample to be representative are worked out.
  3. Participants that made up each stratum are selected using random sampling.
• Strengths of stratified sampling

  • Representative sample because it is designed to accurately reflect the composition of the population. This means generalisation of findings is possible.
• Limitations of stratified sampling

  • Identified strata do not reflect all ways people are different so complete representation of a target population is not possible.
• Opportunity sample

  Given that representative samples of the target population are so hard to obtain researchers decide to select anyone who happens to be willing and available. The researcher takes the chance to ask who is around at the time of the study.

  E.g in the street.
• Strengths of opportunity sample

  • Convenient, much less costly in terms of time and money, because a list of the target population is not required.
• Limitations of opportunity sample

  • Suffer from 2 types of bias. Sample is unrepresentative of target population as it is drawn from a specific area, so findings cannot be generalised to the whole target population. Research also has complete control over the selection of participants and may avoid people they do not like the look of (researcher bias).
• Volunteer sample


  Involves participants selecting themselves to be part of a sample.

  To select a volunteer sample a researcher may place an advert in a newspaper.
• Strengths of volunteer sample

  • Easy, it requires minimal input from the researcher so less time consuming.
  • Participants are more engaged.
• Limitations of volunteer sample

  Volunteer bias

  May attract a certain type of people, one who is curious and more likely to please the researcher, this affects generalisation.