Statistical Surveys 2

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Statistical Surveys 2

• Sampling Methods

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Sampling techniques

• A random sample is a sample drawn in such a way
  that each element of the population has a chance
  of being selected.
• A random sample is likely to be representative of
  the population.

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Simple Random Sample

• A simple random sample is a sample that is
  selected in such a way that each member has the
  same chance of being included in the sample.
• This method is best suited to a reasonably small
  sample frame. It becomes tedious time-consuming
  if the population is large.

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Systematic Random Sample

• Starting at a random point, and sampling every
  nth individual (such as every 15th person on the
  school roll).
• This method is more convenient for a large
  population than the simple random sample.
• This method is likely to give a sample which is
  representative of the population

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Stratified Random Sample

• In a stratified random sample we first divide the
  population into groups which are called strata.
  Then a sample is selected from each strata in
  proportion to the size of the group. The
  collection of all samples from all strata gives
  the stratified random sample.

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Quota sample

• A quota sample is like a stratified sample,
  except that the sample may or may not be chosen
  in a random way.
• In a quota sample, a researcher keeps sampling
  until they have enough of each category being
  investigated.
• Eg all age groups in the NZ health survey

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Cluster Sample

• In cluster sampling the whole population is first
  divided into geographical groups called clusters.
  Each cluster is representative of the population.
  Then a random sample of clusters is selected.
  Finally a random sample is taken from each of the
  selected clusters.

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Convenience sampling

• The sample is taken from the easiest part of the
  population to reach.
• That sample may or may not be representative of
  the population.

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Self-selected sampling

• The sample is open to anyone and people can
  decide whether to be part of it.
• It is very likely that a self-selected sample
  will be biased.