Selecting Research Participants

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Chapter 5

• Selecting Research Participants

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Typically, researchers are interested in
discovering general truths about a population of
individuals.
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Typically, researchers are interested in
discovering general truths about a population of
individuals. A population is the entire set of
individuals that is of interest to the researcher.
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Typically, the population of interest is too
large for each individual to be studied. As a
result the researcher will select a sample of
individuals from the population
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Typically, the population of interest is too
large for each individual to be studied. As a
result the researcher will select a sample of
individuals from the population A sample is a
subset of the population which includes the
individuals that are actually selected to be part
of the study.
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The focus of this chapter is on samples.
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A sample should representative of the population.
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A sample should representative of the
population. A representative sample is one that
displays the same characteristics as the
population.
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A sample which is not representative of the
population is said to be biased.
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Two general methods of selecting samples

• Probability sampling Each individual in the
  population has a specific probability of
  selection. This requires that the entire
  population be known.
• Nonprobability sampling The sample is selected
  based on the availability of individuals to the
  researcher.

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Types of probability sampling

• Simple random sampling Each individual in the
  population has an equal and independent chance of
  being selected.
• Systematic sampling Every nth individual is
  selected after a random start.

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Probability sampling contd.

• Stratified random sampling When the population
  consists of identifiable subgroups, the
  researcher might choose to select equal random
  samples from each subgroup.
• Proportionate stratified random sampling The
  same as stratified random sampling except that
  the researcher randomly selects samples from each
  subgroup that are proportionate to its size.

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Probability sampling contd.

• Cluster sampling The researcher randomly
  selects clusters or preexisting groups.

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Types of nonprobability sampling

• Convenience sampling Researcher selects
  participants who have made themselves available.
• Quota sampling The researcher identifies
  subgroups in the population, then specifies
  quotas for the number of individuals selected
  from each subgroup.