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Do Now

Do Now. Independently answer the following questions in your spiral. What is the difference between a population and a sample? What is the purpose of a sample?

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Do Now

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  1. Do Now Independently answer the following questions in your spiral. • What is the difference between a population and a sample? • What is the purpose of a sample? • True or False: A survey that only polls 15% of a population has an undercoverage bias because 85% of the population is not included. • Defend your answer to #3.

  2. Homework • Continue brainstorming your project ideas. • Finish your “Wandering Proposal” if not done during class.

  3. Agenda • Sampling Concepts Review • Return Chapter 12 Quizzes • Project Intro!! Get Excited!!!  • Project Wandering Proposal • Due @ end of class

  4. Do Now Independently answer the following questions in your spiral. • What is the difference between a population and a sample? • What is the purpose of a sample? • True or False: A survey that only polls 15% of a population has an undercoverage bias because 85% of the population is not included. • Defend your answer to #3.

  5. Sampling

  6. Population of interest: Mint Chocolate Chip Simple Random Sample

  7. Population of interest:All 6 Flavors Stratified Sample

  8. Population of interest:Box of NeapolitanIce Cream Sandwiches Cluster Sample

  9. Voluntary Response Bias • Sample Badly with Volunteers: • In a voluntary response sample, a large group of individuals is invited to respond, and all who do respond are counted. • Voluntary response samples are almost always biased, and so conclusions drawn from them are almost always wrong. • Voluntary response samples are often biased toward those with strong opinions or those who are strongly motivated. • Since the sample is not representative, the resulting voluntary response bias invalidates the survey.

  10. Convenience Bias • Sample Badly, but Conveniently: • In convenience sampling, we simply include the individuals who are convenient. • Unfortunately, this group may not be representative of the population. • Convenience sampling is not only a problem for students or other beginning samplers. • In fact, it is a widespread problem in the business world—the easiest people for a company to sample are its own customers.

  11. Example • You want to know what Muchin senior think about the prom theme. • Convenience Sample: Your friends. • Bias: You friends may all think alike or may just agree with you because you are their friend.

  12. Undercoverage Bias • Sample from a Bad Sampling Frame: • An SRS from an incomplete sampling frame introduces bias because the individuals included may differ from the ones not in the frame. • Undercoverage: • Many of these bad survey designs suffer from undercoverage, in which some portion of the population is not sampled at all or has a smaller representation in the sample than it has in the population. • Undercoverage can arise for a number of reasons, but it’s always a potential source of bias.

  13. Example • You want to know what Muchinstudents (9-12th grades) think about the senior/junior off-campus lunch policy. • You stop every 5th student who comes off of the elevator between 7:00 – 7:30 am. • Which group is under-represented here?

  14. Nonresponse Bias • Watch out for nonrespondents. • A common and serious potential source of bias for most surveys is nonresponse bias. • No survey succeeds in getting responses from everyone. • The problem is that those who don’t respond may differ from those who do. • And they may differ on just the variables we care about.

  15. Example • You want to find out what percentage of people who walk downtown feel stressed / busy. • Who will stop to talk to you? • Will you be able to account for the busy people?

  16. Response Bias • Work hard to avoid influencing responses. • Response bias refers to anything in the survey design that influences the responses. • For example, the wording of a question can influence the responses: • “You agree with me, right?”

  17. Example • The survey can be biased (wording of questions, answer choices, etc.) • The circumstances can influence people’s answers. • Ms. Neal gives you a survey with the following circumstances: • You have to put your name on it. • You have to fill it out in front of her. • It asks you various questions about cheating, skipping, and complaining about teachers. • Will these circumstances affect your response?

  18. Unit 3 Project Experiments and Surveys

  19. Get Creative! • Work individually. • Connect it to your senior project. • Survey and / or experiment. • Next two weeks – calendar of process / deadlines tomorrow.

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