Sampling Methods

• Simple Random Sampling- How to Carry Out:

  Every sampling unit has an equal chance of being selected.
• Simple Random Sampling-Method:

  Use a random number generator- with each item having an identifying number.
• Simple Random Sampling- Advantages:

  • Bias free
  • Easy + cheap to implement
  • Each sampling unit has an equal chance of being selected
• Simple Random Sampling- Disadvantages

  • Not suitable when population size is large
  • Sampling frame needed
• Systematic Sampling- How to Carry Out:

  Required elements are chosen at regular intervals in an ordered list.
• Systematic Sampling-Method:

  i.e. take every kth element where:

  k = pop size (N) / sample size (n)

  starting at random item between 1 and k
• Systematic Sampling- Advantages:

  • Simple + quick to use
  • Suitable for large samples or populations
• Systematic Sampling- Disadvantages:

  • Sampling frame needed
  • Can introduce bias if sampling frame is not random
• Stratified Sampling- How to carry out:

  Population divided into groups (strata) and a simple random sample is carried out in each group.
• Stratified Sampling- When to use:

  When samples are large and population is naturally divided into groups.
• Stratified Sampling- Advantages:

  • Guarantees proportional representation of groups within population
• Stratified Sampling- Disadvantages:

  • Population must be clearly classified
  • Selection within each stratum suffers from the same disadvantages as simple random sampling
• Quota Sampling- How to carry out:

  Population divided into group according to characteristics. A quota of items/people is set to reflect the groups. Interviewer selects the actual sampling unit.
• Quota Sampling- Advantages:

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

  • Non-random sampling introduces bias
  • Population must be divided into groups- costly or inaccurate
  • Increasing scope of study increases number of groups, adding time or expense
  • Non-responses aren't recorded
• Opportunity Sampling- How to carry out:

  Sample taken from people who are available at the time of study who meet the criteria.
• Opportunity Sampling- Advantages:

  • Easy to carry out
  • Inexpensive
• Opportunity Sampling- Disadvantage:

  • Unlikely to provide representative sample
  • Highly dependent on individual researcher