final ppt of sampling
TRANSCRIPT
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Population & Sampling
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Population
Population refer to the entire groupof people,events,or things of interestthat the researcher wishes toinvestigate.
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Element
An element is a single member ofthe population.
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Sampling Frame
The population frame is a listing ofall element in the population fromwhich the sample is drawn
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Sample
A sample is a subset of the
population. it comprise somemember selected from it
Target Population
Sample
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UNIT
A unit is a single member of thesample, just as an element is a singlemember of the population
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Why Sample?
Lower Cost.
Greater Accuracy of result.
Greater speed of data collection.
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What is a Good Sample?
Validity of a sample depends on twoconsideration
Accuracy
Precision
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Steps In Sampling Design
What Is Target Population?
What are the parameter of interest?
What is sampling frame? What is the appropriate sampling
method?
What size sample is needed?
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Normality Of Distribution
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Sampling Method
Probability
Nonprobability
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Probability
Simple Random
Complex Random
Systematic Stratified
Cluster
Double
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Nonprobability
Convenience
Purposive
JudgmentQuota
Snowball
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The Problem Of Non-Responce
You can randomly pick elements from sampling
frame and use them to randomly select people
But you cannot make people respond
Non-response destroys the generalize ability of the
sample. You are generalizing to people who arewilling to respond to surveys
If response is 90% or so, not so bad. But if it is
50%, this is a serious problem
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Errors in sample surveys
Survey results are typically subjectto some error. Total errors can beclassified into sampling errors and
non-sampling errors. The term"error" here includes systematicbiases as well as random errors
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Sampling errors
Sampling errors are induced by the sampledesign. They include:
Selection bias
When the true selection probabilities differfrom those assumed in calculating the results.
Random sampling error
Random variation in the results due to the
elements in the sample being selected atrandom.
http://en.wikipedia.org/wiki/Selection_biashttp://en.wikipedia.org/wiki/Sampling_errorhttp://en.wikipedia.org/wiki/Sampling_errorhttp://en.wikipedia.org/wiki/Selection_bias -
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Non-sampling error
Non-sampling errors are caused by other problems in datacollection and processing. They include:
Overcoverage Inclusion of data from outside of the population.
Undercoverage
Sampling frame does not include elements in the population. Measurement error E.g. when respondents misunderstand a question, or find it
difficult to answer.
Processing error Mistakes in data coding.
Non-response Failure to obtain complete data from all selected individuals.