3.2-3.3 - Samplingdesign Sampling methods Simple random...

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Sampling design Sampling methods Simple random sampling Stratified random sampling Multi-stage sampling Sampling distribution and statistical inference Caution about sampling
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  Convenience sampling : Just ask whoever is around. Example: “Man on the street” survey (cheap, convenient) Which men, and on which street? Ask about gun control or legalizing marijuana “on the street” in Berkeley or in some small town in Idaho and you would probably get totally different answers. Even within an area, answers would probably differ if you did the survey outside a high school or a country western bar. Bias: Opinions limited to individuals present. Sampling methods
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  Voluntary Response Sampling (self selection) : Individuals choose to be involved. These samples are very susceptible to being biased because different people are motivated to respond or not. Often called “public opinion polls”, these are not considered valid or scientific. Bias: Sample design systematically favors a particular outcome.
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CNN on-line surveys: Bias: People have to care enough about an issue to bother replying. This sample is probably a combination of people who hate “wasting the taxpayers money” and “animal lovers.”
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In contrast : Probability or random sampling : Individuals are randomly selected. No one group is over- represented. Random samples rely on the absolute objectivity of random numbers. There are tables and books of random digits available for random sampling.
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