Download Research Methods Lecture 3: Sampling

Research Methods
Sampling
Sampling
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Purpose: To draw enough of something to make your
findings generalizable
Some things to do before conducting a sample
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Consider a census
Evaluate Generalizability
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Sample generalizability: Can the findings from the sample be
generalized to the population from which that sample was
taken?
Cross-population generalizability: Can the findings from one
population be generalized to another slightly different
population?
Assess the diversity of the sample
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Shoot for a representative sample…
Sampling Methods
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Non-probability Samples
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Often used in qualitative research
Probability Samples
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Often used in quantitative research
Nonprobability Sampling
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Why do it?
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Random sampling may not be possible or is
too expensive
May be doing exploratory research
Types of nonprobability samples
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Availability sample
Quota sample
Purposive sample
Snowball sample
Probability Samples
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A means that allows us to know in advance the
likelihood that an element will be selected from
the population
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Relies on random selection
Problems to watch out for when selecting a random
sample:
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An incomplete sampling frame
Failure to obtain an adequate response rate
Random samples and sampling error
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Generally, a random sample has sampling error due to
chance
 Use inferential statistics to calculate sampling error
2 things effect the degree of error due to chance:
 The size of the sample
 The Homogeneity of the population
Probability Sampling Continued
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Importance: Alf Landon vs. Roosevelt
presidential sample
Types of probability samples
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Simple random sample (SRS)
Systematic random sample
Stratified random sample
Cluster sample
Sampling Distributions
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Errors may occur when drawing samples (the
sample is not representative of the
population)
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Two reasons why this can occur
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You make a mistake (systematic sampling error)
Errors due to chance (random sampling error)
Use inferential statistics to determine sampling
error
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Confidence Intervals
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Usually reported at 95%, 99%, & 99.9%
Determining Sample Size
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Usually use about 1,000 - 1,500 for U.S.
population if looking for a simple
description
Usually use up to 2,500 if wanting to
know about something detailed
Locally or regionally, typically a few
hundred