Population and sample
- Population → parameter (usually unknown).
- Sample → statistic (an estimate).
Watch out: a number from the sample is a statistic.
Revision sheet
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Sampling methods
SRS · stratified (some from every group) · cluster (all from some groups) · systematic (every th).
Watch out: proportional allocation in stratified samples.
Bias
Voluntary response, convenience, undercoverage, nonresponse, response bias.
Watch out: a bigger biased sample is still biased.
Studies
- Observational: record. Experiment: assign.
- Confounding hides cause in observational data.
Watch out: association is not causation.
Experimental design
Compare · randomize · replicate · block. Treatments = combinations of factor levels.
Watch out: blocks (experiments) vs strata (samples).
Scope of inference
- Random sampling → generalize.
- Random assignment → cause.
Watch out: you need both for a causal claim about a population.
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