maths

• simple random sampling


  every sample has an equal chance of being selected (use random number generator)

  pros: bias free, easy, cheap

  cons: sampling frame needed
• systematic sampling


  required elements chosen at regular intervals in ordered list

  pros: simple, quick, suitable for large samples

  cons: sampling frame needed, can introduce bias
• stratified sampling


  population divided into groups, simple random sample carried out in each group -> used when sample is large

  pros: reflects population structure, guaranteed proportional representation of groups

  cons: same as simple random sampling
• quota sampling


  population divided into groups based on characteristic (eg age group), a quote is set to reflect groups proportion in the whole population

  pros: no sampling frame, quick, easy, inexpensive

  cons: non random -> bias, population must be divided into groups (can be costly & inaccurate)
• opportunity / convinience sampling


  sample from people available at time of study

  pros: easy and inexpensive

  cons: unlikely to provide representative sample