17352 08ppt

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Chapter EightSampling: Who, What and How Many?

Key Concepts• Population. • Sample. • Census. • Parameters.• Probability sampling -

▫random, stratified random, systematic, multi-stage cluster

• Nonprobability sampling - ▫convenience, purposive, quota, network

• Sampling frames• Sampling error

Sampling Terms•Population - every person in the group

you are studying.•Sample - part of a population selected for

study.•Census - study of entire population.•Parameters - describe a population.•Statistics - describe a sample.•Sampling frame - list from which a sample

is drawn.

The Two Major Types of Sampling

•Probability – ▫Random selection. Every unit has an

equal chance of being selected.

•Nonprobability – ▫Sampling is based on judgment by the

researcher.

Nonprobability Sampling

•Convenience

•Volunteer

•Purposive or judgment

•Quota

•Snowball or network

Probability Sampling

•Simple random -▫Lottery, random numbers.▫http://www.random.org▫http://www.randomizer.org.

•Systematic - ▫Stratified random.▫Multistage cluster.

Convenience Sample

First ten items selected arbitrarily

Stratified Random Sample

40% of the sample is 30% of the sample is 20% of the sample is 10% of the sample is

Percentage of each groupin sample is the same as thepercentage of that group in population.

Random sampling takes place within each group.

Systematic Sample

Random starting point selected.

Then every nth unit.

(n = 6 in this example)

Sampling Error

•The degree to which the sample statistics differ from the parameters of the universe or population from which it was selected.

•Margin of error, confidence level and size of sample are related.

Sample Size•Depends on –

▫Purpose of the research.▫Desired level of statistical confidence.▫Homogeneity of the population.▫In practice, the time and resources

available for research.

Standard Error (%), Probability of Answers and Sample Size

Probability of Answers

Size of Sample

90:10 50:50

10 9.5% 15.8%

100 3.0% 5.0%

1000 0.95% 1.6%

Sampling Frame Issues

•Postal Sampling▫may be dated▫cannot access the transient & homeless

•Phone Sampling▫problems accessing mobile numbers▫consumer resistance to calls

•Internet Sampling▫difficulties generalizing from Internet

population to general population.

Chapter Summary

•Census - study of an entire population.

•Sample - part of population chosen for study.

•Probability sampling – random basis.

•Nonprobability sampling – judgment of researcher

•Larger sample sizes reduce sampling error.

•Statistical formulae give us sample size for a given margin of error, or vice versa.

Vocabulary Review

Web Resources

• Council of American Survey Research Organizations -

http://www.casro.org • Research Randomizer - http://randomizer.org• StatPac -

http://www.statpac.com/surveys/sampling.htm• Survey Sampling International - www.surveysampling.com• WebSM - http://www.websm.org• World Association for Public Opinion Research -

http://www.unl.edu/wapor

17352 08ppt

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