Sample Surveys

Sample Surveys - Lecture Unit 3 Sample Surveys Producing...

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Lecture Unit 3 Sample Surveys Producing Valid Data “If you don’t believe in random sampling, the next time you have a blood test tell the doctor to take it all.”
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The election of 1948 The Predictions The Candidates Crossley Gallup Roper The Results Truman 45 44 38 50 Dewey 50 50 53 45
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Beyond the Data at Hand to the World at Large We have learned ways to display, describe, and summarize data, but have been limited to examining the particular batch of data we have. We’d like (and often need) to stretch beyond the data at hand to the world at large. Let’s investigate three major ideas that will allow us to make this stretch…
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3 Key Ideas That Enable Us to Make the Stretch
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Idea 1: Examine a Part of the Whole The first idea is to draw a sample. We’d like to know about an entire population of individuals, but examining all of them is usually impractical, if not impossible. We settle for examining a smaller group of individuals—a sample —selected from the population.
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Examples 1. Think about sampling something you are cooking —you taste (examine) a small part of what you’re cooking to get an idea about the dish as a whole. 2. Opinion polls are examples of sample surveys, designed to ask questions of a small group of people in the hope of learning something about the entire population.
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Convenience sampling : Just ask whoever is around. – Example: “Man on the street” survey (cheap, convenient, often quite opinionated or emotional => now very popular with TV “journalism”) 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 : 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. Ann Landers summarizing responses of readers 70% of (10,000) parents wrote in to say that having kids was not worth it—if they had to do it over again, they wouldn’t. Bias : Most letters to newspapers are written by disgruntled people. A random sample showed that 91% of parents WOULD have kids again.
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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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Example: hospital employee drug use Why might this result in a biased sample?
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This note was uploaded on 03/10/2011 for the course BUS 350 taught by Professor Reiland during the Spring '08 term at N.C. State.

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Sample Surveys - Lecture Unit 3 Sample Surveys Producing...

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