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August 06, 2015

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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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August 06, 2015

Aug 8­9:23 AM

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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