sampling

• Population - the large group of people that a researcher is interested in studying
• Sample - it is usually not possible to include all members of the population in the study, so a smaller group is selected
• Generalisation - the sample that is drawn should be representative of the population so generalisations can be made
• Bias - most samples are biased in that certain groups may be over or under represented
• Random sample

  Every person in the target population has an equal chance of being selected. This is done using the lottery method which is where all members of a target population are given a number and placed in a hat or computer randomiser. Evaluation :

  1. Strength - potentially unbiased. This means EVs/CVs are controlled. Enhances internal validity
  2. Limitation - Time consuming and may not work. Complete list of population is hard to get. Also some participants may refuse to take part.
• Systematic sample

  Participants are selected using a set pattern. This is done by selecting every nth person from a list of the target population. Evaluation :

  1. Strength - Unbiased. The first item selected is selected at random which is an objective method.
  2. Limitation - Time and effort. A complete list of the population is required so they may aswell use random sampling.
• Stratified sample

  Strata - sample reflects proportions of people in certain subgroups within a population. This is done by identifying subgroups and then the relative subgroup percentages are reflected in the sample. Evaluation :

  1. Strength - Representative method. The characteristics of the target population are represented. Generalisability is more likely than other methods.
  2. Limitation - Stratification is not perfect. Strata cannot reflect all the ways in which people are different. Complete representation is not possible.
• Opportunity sample

  People who are simply most available . This is done by asking people nearby. Evaluation :

  1. Strength - quick method. Opportunity sample is convenient as you just make use of the people who are closest. This makes it cheaper and one of the most popular sampling methods.
  2. Limitation - Inevitably biased. The sample is unrepresentative of the target population as it is drawn from a very specific area. This means that the findings cannot be generalised.
• Volunteer sample

  In a volunteer sample, participants select themselves. This is done through advertising. Evaluation :

  1. Strength - Participants are willing. Participants have selected themselves and know how much time and effort is involved. This is likely to engage more than people stopped in the street.
  2. Limitation - Volunteer bias. Participants may share certain traits e.g. helpful. Respond to cues and generalisation limited.