Sampling

• Population 


  The large group of people that a researcher is interested in studying.

  e.g. college students from the North West
• Sample


  It is usually not possible to include all the members of the population in a 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


  Most samples are biased in that certain groups may be over / under represented
• Random sample 


  Equal chance; every person in the target population has an equal chance of being selected

  How?; lottery method - all members of the target population are given a number and placed into a hat or computer randomiser used

  + potentially unbiased; CVs / EVs are controlled = enhance internal validity

  -- time consuming and may not work; complete list of the population is difficult to obtain & some ppts may refuse to take part
• Systematic Sample


  Using a set system; participants are selected using a set 'pattern' (sampling frame)

  e.g. alphabetical order

  How?; every nth person is selected from a list of the target population

  
  

  + unbiased; the first item is usually selected at random

  --time & effort; a complete list of the population is required = may as well use random sampling
• Stratified Sample


  Strata; sample reflects proportions of people in certain subgroups (strata) within a population

  ;How?; subgroups (or 'strata') are identified e.g. age / gender - the relative percentages of the subgroups in the population are reflected in the sample

  + representative method; the characteristics of the target population are represented - generalisability more likely than other methods

  -- stratification not perfect; strata cannot reflect all the ways in which people are different = complete representation is not possible
• Opportunity Sample 


  Most available; people who are simply most available i.e the ones that are closest / easiest to obtain

  How?; Ask people nearby

  + Quick method; convenient as people are able to make use of the people who are closest = cheaper option

  --inevitably biased; sample is unrepresentative of the target population as it is drawn from a very specific area ... findings cannot be generalised
• Volunteer Sample


  Self-selecting; participants select themselves

  How?; Advertise (place in advert / newspaper)

  + participants are willing; selected selves so know how much time and effort is involved = more likely to engage than those selected off of the street

  -- volunteer bias; participants may share certain traits (e.g. want to be helpful) = respond to cues and generalisation is limited