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4.2 What are Good and Poor Ways to Sample?
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Learning Objectives:
• Sampling Frame & Sampling Design• Simple Random Sample• Random number table• Margin of Error• Convenience Samples • Types of Bias in Sample Surveys• Key Parts of Sample Survey
artstuff.typepad.com
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Sampling Frame & Sampling Design
• Sampling frame – list of subjects in (hopefully total) population• Sampling design determines how sample is selected
www.nedarc.org
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Simple Random Sampling (SRS)
• Random Sampling – best way to get representative sample• Simple Random Sample – each possible sample of set size n has equal chance of being selected
www.youthnoise.com
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SRS: Table of Random Numbers
Pg. A6 of text
• Number subjects• Select numbers from random number table or random number generator• Include subjects with random numbers selected
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Margin of Error for Population Percentages
• Margin of Error – how well sample predicts population• With n subjects in SRS, margin of error is about
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Example: Margin of Error
A survey result states: “The margin of error is plus or minus 3 percentage points”
2010 British PM Electioni.telegraph.co.uk
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Convenience Samples: Poor Ways to Sample
• Convenience Sample: survey sample that’s easy to get• Unlikely to represent population• Often severe biases• Results apply only to observed subjects
www.revophth.com
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Convenience Samples: Poor Ways to Sample
Volunteer Sample: most common convenience sample where subjects volunteer – not representative
www.stlouiscenterforclinicalresearch.com
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Types of Bias in Sample Surveys
Bias: Favoring parts of population• Sampling Bias: from sampling method (e.g., nonrandom samples)• Nonresponse bias: some subjects cannot be reached or decline• Response bias: subject gives incorrect response or question is misleading
4.bp.blogspot.com
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Key Parts of a Sample Survey
• Identify entire population• Construct sampling frame• Random sampling design to get n subjects from frame• Avoid sampling bias
Crossstitch Samplerwww.vam.ac.uk
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