Sampling Techniques

• What are sampling techniques?

  They are used to choose a representative sample from the target group.
• Random sampling

  Gives every member of the target group an equal chance of being selected for the sample (e.g. by assigning a number to each member, and then selecting from the pool using a random number generator)

  
  

  Strengths:

  • each PP has the same probability of being selected so there's a reasonable chance of achieving a representative sample

  Weaknesses:

  • it can be impractical
  • minority subgroups within population may not be present in the sample
• Systematic sampling

  Selecting from a target group, e.g. every fourth person in a list could be used in the sample.

  Strengths:

  • unbiased chance of gaining a representative sample (if list order has been randomised)

  Weaknesses:

  • if the list has been assembled in any other way, bias may be present
• Stratified sampling

  Dividing the target group into sections that each show a key characteristic which should be present in the final sample, then each section is sampled individually

  Strengths:

  • avoids the problem of misrepresentation sometimes caused by purely random sampling

  Weaknesses:

  • takes more time and resources to plan
  • if the strata are not accurately defined or if individuals are misclassified into incorrect strata, the results can be biased and misleading
• Opportunity sampling

  PPs who are accessible are targeted, e.g. employees from a conveniently located employer near the laboratory could be selected for the sample group

  Strengths:

  • easy and inexpensive to carry out

  Weaknesses:

  • consequent sample may not be representative as it could be subject to bias (they may possess similar qualities, making the group unrepresentative)
• Volunteer sampling

  The sample consists of people who have volunteered to be in the study

  Strengths:

  • achieves a large sample size through reaching a wide audience, for example with online advertisements

  Weaknesses:

  • the PPs that respond to the call for volunteers may all display similar characteristics (such as being more trusting or cooperative than those who did not apply) which increases the chances of an unrepresentative sample.