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Sampling

Sampling. 12/4/2012. Readings. Chapter 8 Correlation and Linear Regression (Pollock) ( pp 199- 206) Chapter 6 Foundations of Statistical Inference (Pollock) ( pp 122-135) . Final Exam. SEC 1 December 12 th (Wednesday) 1:30 pm - 3:30 pm SEC 2 December 11 th (Tuesday)

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Sampling

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  1. Sampling 12/4/2012

  2. Readings • Chapter 8 Correlation and Linear Regression (Pollock) (pp 199- 206) • Chapter 6 Foundations of Statistical Inference (Pollock) (pp 122-135)

  3. Final Exam • SEC 1 • December 12th (Wednesday) • 1:30 pm - 3:30 pm • SEC 2 • December 11th (Tuesday) • 1:30 pm - 3:30 pm

  4. Final Paper • Due 12/7/2012 by 11:00AM- Doyle 226B • Turnitin Copy by 11:59PM on 12/7

  5. Opportunities to discuss course content

  6. Office Hours For the Week • When • Wednesday 8-4 • Thursday 10-12 • And by appointment

  7. Course Learning Objectives • Students will learn the basics of polling and be able to analyze and explain polling and survey data • Students will learn the basics of research design and be able to critically analyze the advantages and disadvantages of different types of design.

  8. Sampling After we write the survey, we have to select people!

  9. Why? Non-Probability Samples

  10. Quota Samples • A Type of Judgment Sample • Break the nation into groups • Pick a certain number/quota from each group • Stop when you have filled up your quota

  11. The Death of Quota Sampling: 1948 • We used to use these for national polls • George Gallup thrived on these. • In 1948 he predicts that Thomas Dewey of New York would defeat Harry Truman

  12. Why Gallup was Wrong • It was a close election • The electorate diversified (missed out on groups) • They filled up quotas with easy targets • They stopped polling

  13. Snowball Sample • one becomes two, becomes four, becomes 8 • Difficult to Reach Populations • Background Checks

  14. Looking through A Parent’s eyes The Most Beautiful Kids Ever Internal Polling

  15. The Laws of Sampling • if cost is not a major consideration it is better to collect data for ones target population than for a sample thereof • if cost dictates that a sample be drawn, a probability sample is usually preferable to a nonprobability sample. • The Law of Large Numbers • The accuracy of estimates is expressed in terms of the margin or error and the confidence level. • all probability samples yield estimates of the target population.

  16. Probability Sampling

  17. Rules on Sampling • if cost dictates that a sample be drawn, a probability sample is usually preferable to a nonprobability sample. • The Law of Large Numbers

  18. Collecting a sample • Population • Sampling Frame • The Sample itself

  19. Probability Samples • Ensure that every unit in the population has an equal chance of being selected • In a simple random sample all elements in the population can be selected (SRS) • This involves having a full list of everyone! • We cannot do a SRS of the United States

  20. The best that we can hope for is that every unit in the sampling frame has an equal chance of being selected

  21. How to do it- Simple Way Random Number Table The Lottery Method

  22. The Law of Large Numbers • Smaller samples cause greater error. • The larger the sample size, the greater the probability that our sample will represent the population.

  23. All probability samples yield estimates of the target population

  24. Two Things that Deal With the Stars Astronomy Astrology

  25. Polling is Science (Astronomy) • Polls are right more than they are wrong • We especially love them when it favors our candidates.

  26. Polling is Random (Astrology) • It is not an exact science, there is error in every poll. • Polls Don’t Vote, People Vote • We like it less when it doesn’t favor our candidate

  27. Same Election, Different Results

  28. Different Questions Perhaps? • If the election were held today, would you vote for Barack Obama or Mitt Romney? • If the election were held today, would you vote for Mitt Romney or Barack Obama? • If the election were held today, would you vote for Democrat Barack Obama or Republican Mitt Romney? • If the election were held today, would you vote for Republican Mitt Romney or Democrat Barack Obama? • If the election were held today, for whom would you vote?

  29. More Likely a different sample

  30. Polling is 95% Science and 5% Astrology Sampling error

  31. The accuracy of estimates is expressed in terms of the margin or error and the confidence level

  32. The Confidence Level • The Confidence Level- can we trust these results? • Surveys use a 95% confidence interval that the results will fall within the margin of error • There is a 5% (1 out of 20) chance that the results will fall outside this range and produce wacky findings. • This error often appears when you keep asking the same questions again and again

  33. The Margin of Error • Margin of Error • A floating range above and below the estimate. • Large Samples= Less Error

  34. What else determines sampling error • Non-response rate • Variability • Bias

  35. How Can a Survey of 1000 People Represent Millions of Voters? • Responses Cancel each other out • No New opinions are added

  36. Its Logarithmic

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