Sampling

• Sample


  A subset of data taken from the whole population that should represent the whole population
• Population


  The whole set of data that is of interest
• Parameter

  Data values that can be used to summarise the population, for example the Mean
• Sampling Unit


  Each individual thing in the population that could be sampled
• Sampling Frame

  Sampling units of a population are individually numbered or named to form a list called the sampling frame
• Simple Random Sampling- each number has a known equal chance of being selected
• Simple Random Sampling


  1. Number the population
  2. Use a random number generator to generate a random two digit number between the class width
  3. Ignore repeats or numbers out of range and continue until you have picked the sample
  4. These numbers correspond to your population
• Advantages of Simple Random Sampling


  • Bias free
  • Easy and cheap to carry out
• Disadvantages of Simple Random Sampling

  • Not suitable when population size is large
  • Sampling frame needed
• Systematic Sampling


  • Take every kth element from an ordered list
  • Starting at a random item between 1 and k
• Advantages of Systematic Sampling


  • Suitable for large samples
  • Sampling frame needed
• Disadvantages of Systematic Sampling

  • Can introduce bias if sampling frame not random
• Stratified Sampling


  • Population divided into distinct and non-overlapping groups
  • Sampling done within each group to reflect population structure
• Advantages of Stratified Sampling

  • Reflects population structure
  • Guarantees proportional representation of population
• Disadvantages of Stratified Sampling


  • Population must be clearly classified into distinct strata
  • Selection within each stratum suffers disadvantages of simple random sampling
• Quota Sampling


  • Population divided into groups according to characteristic
  • A quota of items/people in each group is set to try and reflect the group's proportion in the whole population
  • Interviewer selects the actual sampling units
• Advantages of Quota Sampling

  • Allows small sample to still be representative of population
  • No sampling frame required
  • Quick, easy and inexpensive
  • Allows for easy comparison between different groups in population
• Disadvantages of Quota Sampling

  • Non-random sampling can introduce bias
  • Population must be divided into groups, which can be costly or inaccurate
  • Increasing scope of study increases number of groups adding time/expense
  • Non-responses are not recorded
• Opportunity/Convenience Sampling


  • Sample taken from people who are available at time of study who meet criteria
  • Easy to carry out
  • Inexpensive
• Disadvantages of Opportunity/Convenience Sampling

  • Unlikely to provide a representative sample
  • Highly dependent on individual researcher
• Cluster Sampling


  • Population divided into groups-called clusters
  • Select clusters to conduct your sample with
• Advantages of Cluster Sampling

  • Don't need a sampling frame, or to name or number all the population
  • Don't have to rank whole population- less data to handle
• Disadvantages of Cluster Sampling

  • Non-random sampling can introduce bias
  • Population must be divided into groups, which can be costly or inaccurate
  • Increasing scope of study increases number of groups adding time/expense
  • May not be representative