26.3 Populations and samples

• The inclusion and exclusion criteria define the sample
  and the population to which the study applies

• Ideally, the sample is a random subset of the population

  • Any member of the population is equally likely to be represented

• To the extent that the sample is not random,
  the inferences will be biased

  • If you pole a Republican district, the result will
    be biased towards the Republican party