research methods lecture 27. nonprobability and probabilitysampling
TRANSCRIPT
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Major Alternatives to Sampling
ProbabilityNon-probability
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ProbabilityEvery element in the population has a known nonzero
probability of selection.Used when representativeness is crucial for wider
gerenalizability.Can measure sampling error.
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Non-ProbabilityProbability of any element being chosen is unknown.
Arbitrary – personal judgment.Time and other factors more crucial than generalizability. Cannot measure sampling error.Projecting data beyond sample is statistically
inappropriate.
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Convenience SamplingAlso called haphazard or accidental or grab
sampling.Obtaining units who are most conveniently available.Person-on-the street interviewed by TV people.
Personal judgment.May not be representative. Least reliable.Used during exploratory phase
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Purposive SamplingResearcher lays down the criteria for the subjects.Also called as judgmental or expert opinion sample.
Selection depends upon the judgment of selector.
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Quota SamplingProcedure that ensures certain characteristics of a
population sample will be represented. Fix the quota. Use convenience sampling.
Fieldworker bias in selection.
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Can be considered as proportionate stratified sampling – but on convenience basis.
Speed of data collection, lower cost, and convenience.
Ensures the inclusion of a subset when it is under-represented.
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Snowball SamplingAlso called: network, chain referral, or reputational
sampling.Analogy of snowball.Begins with one/few, spreads out on the basis of
links.Useful where respondents are difficult to identify.
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Sequential Sampling
Similar to purposive sampling.In purposive – get every possible case.In sequential – continuous evaluation of data. Stop
when no new information is coming.
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Theoretical SamplingResearcher selects cases based on his own insight.Used in observational studies
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RESEARCH METHODS
Lecture 27