Sampling

• What is a population in statistics?

  The whole set of items of interest
• What is raw data?

  Information obtained from a population
• What does a census do?

  Measures every member of a population
• What is a sample in statistics?

  A selection of observations from a population
• What are the advantages and disadvantages of a census?

  Advantages:

  • Results should be completely accurate

  
  

  Disadvantages:

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

  Advantages:

  • Less time consuming and cheaper
  • Fewer people have to respond
  • Less data needs to be processed

  
  

  Disadvantages:

  • Data may not be as accurate
  • Sample may not be large enough for small subgroups
• What are individual units of a population called?

  Sampling units
• What is a sampling frame?

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

  Each member has an equal chance of selection
• What is simple random sampling?

  Every sample of size $n$ has an equal chance
• How can a simple random sample be taken using a calculator?

  Generate 12 random numbers between 1-100
• How can lottery sampling be conducted?

  Draw names from identical cards in a hat
• What are the advantages and disadvantages of simple random sampling?

  Advantages:

  • Free of bias
  • Easy and cheap for small samples
  • Known and equal chance of selection

  
  

  Disadvantages:

  • Not suitable for large samples
  • Requires a sampling frame
• What is quota sampling?

  Sample reflects characteristics of the population
• What are the advantages and disadvantages of quota sampling?

  Advantages:

  • Small sample can be representative
  • No sampling frame required
  • Quick, easy, and inexpensive

  
  

  Disadvantages:

  • Can introduce bias
  • Costly or inaccurate group division
  • Non-responses not recorded
• What is opportunity sampling?

  Sample taken from available people fitting criteria
• What are the advantages and disadvantages of opportunity sampling?

  Advantages:

  • Easy and inexpensive

  
  

  Disadvantages:

  • Unlikely to provide representative results
  • Highly dependent on individual researcher
• What are quantitative variables?

  Variables associated with numerical observations
• What are qualitative variables?

  Variables associated with non-numerical observations
• What is a continuous variable?

  A variable that can take any value in a range
• What is a discrete variable?

  A variable that can only take specific values
• What are the components of a grouped frequency table?

  • Class boundaries show max and min values
  • Midpoint is the average of class boundaries
  • Class width is the difference between boundaries
• What is systematic sampling?

  Elements chosen at regular intervals from a list
• How is systematic sampling conducted with a population of 100?

  Choose every fifth person from the list
• What are the advantages and disadvantages of systematic sampling?

  Advantages:

  • Simple and quick to use
  • Suitable for large samples

  
  

  Disadvantages:

  • Requires a sampling frame
  • Bias if sampling frame is not random
• What is stratified sampling?

  Population divided into mutually exclusive strata
• How is stratified sampling conducted?

  1. Divide population into strata
  2. Calculate number needed from each stratum
  3. Number workers in each group
  4. Use random number generator for selection
• What are the advantages and disadvantages of stratified sampling?

  Advantages:

  • Sample reflects population structure
  • Proportional representation of groups

  
  

  Disadvantages:

  • Must classify population into strata
  • Same disadvantages as simple random sampling