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Finishing Bias

Finishing Bias. Bias. The sampling method is biased if it systematically favors certain outcomes. Exam Tip: Always tell which way it is biased . Selection Bias. The method of selecting the sample systematically excludes some part of the population of interest.

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Finishing Bias

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  1. Finishing Bias

  2. Bias The sampling method is biased if it systematically favors certain outcomes. Exam Tip: Always tell which way it is biased.

  3. Selection Bias • The method of selecting the sample systematically excludes some part of the population of interest. • Example – the rectangles lab voluntary response

  4. Measurement / Response • Method of observation tends to produce values that systematically differ from the true value in some way. • Measurement – improperly calibrated scale, the string activity • Response – Improperly worded questions

  5. Nonresponse bias • Responses are not actually obtained from all individuals selected for inclusion in the sample. • Example – Failure to return polls

  6. What type of Bias • Bill is assigned by his editor to determine what most Americans think about a new law that will place a federal tax on all modems and computers purchased. The revenues from the tax will be used to enforce new online decency laws. Bill, being technically inclined, decides to use an email poll. In his poll, 95% of those surveyed opposed the tax. Bill was quite surprised when 65% of all Americans voted for the taxes.

  7. What type of Bias • The United Pacifists of America decide to run a poll to determine what Americans think about guns and gun control. Jane is assigned the task of setting up the study. To save mailing costs, she includes the survey form in the group's newsletter mailing. She is very pleased to find out that 95% of those surveyed favor gun control laws and she tells her friends that the vast majority of Americans favor gun control laws.

  8. What type of Bias • Large scale polls were taken in Florida, California, and Maine and it was found that an average of 55% of those polled spent at least fourteen days a year near the ocean. So, it can be safely concluded that 55% of all Americans spend at least fourteen days near the ocean each year

  9. Simple Random Sample. • Good sampling technique. • A simple random sample (SRS) of size n consists of n individuals from the population chosen in such a way that every set of n individuals has an equal chance to be the sample actually selected.

  10. Choosing a SRS…using a Random Number Chart • Be specific about how you select • Indicate the stopping rule • Tell whether you sample with or without replacement • Use labels to identify subjects selected to be in the sample

  11. We are planning an article on family-friendly places to stay over spring break at a nearby beach town. The editors intend to call 4 randomly chosen hotels to ask about their amenities for families with children. They have an alphabetized list of all 28 hotels in the town. 69051 64817 87174 09517 84534 06489 87201 97245 88221 22356 77183 88725

  12. Activity • Which rectangle would you choose if you used the random number table? (Table D – back of the book) • How would you do it?

  13. Homework • Worksheet

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