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
