introducing communication research 2e © 2014 sage publications chapter eight sampling: who, what...
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Introducing Communication Research 2e © 2014 SAGE Publications
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
Introducing Communication Research 2e © 2014 SAGE Publications
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.
Introducing Communication Research 2e © 2014 SAGE Publications
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.
Introducing Communication Research 2e © 2014 SAGE Publications
Nonprobability Sampling
•Convenience
•Volunteer
•Purposive or judgment
•Quota
•Snowball or network
Introducing Communication Research 2e © 2014 SAGE Publications
Probability Sampling
•Simple random -▫Lottery, random numbers.▫http://www.random.org▫http://www.randomizer.org.
•Systematic - ▫Stratified random.▫Multistage cluster.
Introducing Communication Research 2e © 2014 SAGE Publications
Convenience Sample
First ten items selected arbitrarily
Introducing Communication Research 2e © 2014 SAGE Publications
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.
Introducing Communication Research 2e © 2014 SAGE Publications
Systematic Sample
Random starting point selected.
Then every nth unit.
(n = 6 in this example)
Introducing Communication Research 2e © 2014 SAGE Publications
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.
Introducing Communication Research 2e © 2014 SAGE Publications
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.
Introducing Communication Research 2e © 2014 SAGE Publications
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%
Introducing Communication Research 2e © 2014 SAGE Publications
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.
Introducing Communication Research 2e © 2014 SAGE Publications
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.
Introducing Communication Research 2e © 2014 SAGE Publications
Vocabulary Review
Introducing Communication Research 2e © 2014 SAGE Publications
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
Introducing Communication Research 2e © 2014 SAGE Publications