statistics

• What is a population?

  The whole set of items that are of interest.
• What is a census?

  It measures/observes every member of the population.
• What is a sample?

  A selection of observations taken from a subset of the population and used to find out information about the population as a whole.
• What are the advantages of a census?

  • Results should be completely accurate.
• What are the disadvantages of a census?

  • Time consuming and expensive
  • Cannot be used when testing destroys process
  • Hard to process large quantities of data
• What are the advantages of a sample?

  • Quicker and its cheaper
  • Fewer people have to respond
  • Less data needs to be processed
• What are the disadvantages of a sample?

  • Data may not be as accurate
  • Sample may not be large enough to give information about small subgroups of the population
• What is a sampling frame?

  A list of sampling units named and numbered.
• What is simple random sampling?

  A simple random sample of size n is one where every sample of size n has an equal chance of being selected.
• What are the advantages of simple random sampling?

  • Free of bias
  • Easy and cheap for small samples and populations
  • Each sampling unit has a known and equal chance of selection
• What are the disadvantages of simple random sampling?

  • Not suitable for large samples and populations
  • Sampling frame is needed
• What is systematic sampling?

  The required elements are chosen at regular intervals from an ordered list.
• What are the advantages of systematic sampling?

  • Simple and quick to use
  • Suitable for larger samples and populations
• What are the disadvantages of systematic sampling?

  • A sampling frame is needed
  • Bias introduced is the sampling frame is not random
• What is stratified sampling?

  The population is divided into mutually exclusive strata and a random sample is taken from each.
• What are the advantages of stratified sampling?

  • Sample accurately reflects the population structure
  • Proportional representation of groups within the population
• What are the disadvantages of stratified sampling?

  • Population must be clearly classified into distinct strata
  • Same disadvantages as simple random sampling within each stratum.
• What is quota sampling?

  An interviewer or researcher selects a sample that reflects the characteristics of the whole population.
• What are the advantages of quota sampling?

  • Allows a small sample to still be representative of the population.
  • No sampling frame required.
  • Quick, easy and inexpensive.
  • Easy comparison between different groups within a population.
• What are the 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, which adds time and expenses.
  • Non-responses not recorded.
• What is opportunity sampling?

  A sample is taken from people who are available at the time of study and who fits the criteria you are looking for.
• What are the advantages of opportunity sampling?

  • Easy
  • Cheap
• What are the disadvantages of opportunity sampling?

  • Unlikely to provide a representative result.
  • Highly dependent on individual researchers.