Sampling

• population

  the large group of people that a researcher is interested in studying, for example college students from the north west.
• sample

  it is usually not possible to include all members of the population in the study, so a smaller group is selected- the sample.
• generalisation

  the sample that is drawn should be representative of the population so generalisations can be made.
• bias

  the majority of samples are biased in that certain groups may be over- or under-represented
• opportunity sample

  people who are simply the most available, i.e. the ones who are nearest/easiest to obtain.
• how to you obtain an opportunity sample?

  ask people nearby, e.g. ask the students in your class to take part or ask people who walk past you at a shopping centre.
• strength of an opportunity sample

  quick method. opportunity sampling is convenient because you just make use of the people who are closest.
• limitation of an opportunity sample

  inevitably biased. the sample is unrepresentative of the target population as it is drawn from a very specific area, such as one street in one town. this means that the findings cannot be generalised.
• volunteer sample

  self-selecting, in a volunteer sample, participants select themselves.
• how is a volunteer sample obtained?

  advertise. for example, place an ad in a newspaper or on a noticeboard and participants come to you.
• strength of a volunteer sample

  participants are willing. participants have selected themselves and know how much time and effort is involved. likely to engage more than people stopped in the street.
• limitation of a volunteer sample

  likely to be a biased sample. participants may share certain traits, e.g. keen and curious. generalisation limited due to volunteer bias.
• random sample

  equal chance. every person in the target population has an equal chance of being selected.
• how is a random sample obtained?

  lottery method. all members of the target population are given a number and placed in a hat or tombola.
• strength of a random sample

  potentially unbiased. the researcher has no influence over who is selected. free from researcher bias.
• limitation of a random sample

  representation not guaranteed. still possible that a random method may produce a biased sample. limits ability to generalise.
• systematic sample

  sampling frame. participants are selected using a set 'pattern' (sampling method).
• how is a systematic sample obtained?

  every nth person is selected from a list of the target population.
• strength of a systematic sample

  unbiased. the first item is usually selected at random. objective method.
• limitation of a systematic sample

  time and effort. a complete list of the population is required. may as well use random sampling.
• stratified sample

  participants are selected according to their frequency in the target population.
• how is a stratified sample obtained?

  subgroups (or 'strata') are identified, such as gender or age groups. the relative percentages of the subgroups in the population are reflected in the sample.