30 October 2013


Why sample?
  • To make an inference about a population
  • Studying entire pop is impractical or impossible

Example of sampling
  • Estimate the proportion of adults, ages 18-65, in Port Elizabeth that have type 2 diabetes
  • Select a sample from which to estimate the proportion
  • Population:  adults aged 18-65 living in Port Elizabeth
  • Inference:  proportion with type 2 diabetes

Probability sampling 
  • Each individual has known (non-zero) probability of selection
  • Precision of estimates can be quantified

Non-probability sampling
  • Cheaper, more convenient
  • Quality of estimates cannot be assessed
  • May not be representative of population

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