Sampling

• Random sample is when every person in the target population has an equal chance of being selected. This can be done by gathering the names of the whole population and placing them in a hat or a computer randomiser is used.

  
  

  +) This eliminates the possibility of bias and that confounding/extraneous variables are controlled.

  
  

  -) A complete list of names is time-consuming and may not work because some participants may refuse to take part.
• Systematic sampling is when a pattern is used to select participants. (e.g. every nth person).

  
  

  +) It's an unbiased method as it randomly selects participants. I.E. its objective.

  
  

  -) A complete list of the population is required and that takes time and effort. And at that point, you might as well use random sampling.
• Stratified sampling is when the sample reflects the proportions of different subgroups in a population. This is done by identifying different groups (e.g. age) and the percentage of that subgroup in the whole population is reflected in the sample.

  
  

  +) It's a very representative method as it takes into account everyone, making it more generalisable.

  
  

  -) Strata cannot reflect all the ways in which people are different, meaning that complete representation is not possible.
• Opportunity sample is when people who are most available are used in the sample. e.g. ask people nearby.

  
  

  +) Quick and convenient method because you make use of the people are closest, making it cheaper then the other methods.

  
  

  -) The sample is unrepresented of the target population as its created from a specific area, meaning that the findings cannot be generalised.
• Volunteer sample is when the participants select themselves, usually via adverts.

  
  

  +) Because participants selected themselves it means they understand that they need to put their time and effort into the study leading to them being more likely to engage then just random people on the street being picked.

  
  

  -) The sample suffers from volunteer bias because participants may share certain traits (e.g. wanting to be helpful therefore why they volunteered).