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ERRORS IN SAMPLING

ERRORS IN SAMPLING. Lesson #10 OCTOBER 5/6, 2011. Errors in Sampling. Sampling Errors Random sampling error – expressed as the margin of error. Errors in Sampling. Sampling Errors Bad sampling methods – voluntary response samples, convenience samples. Errors in Sampling.

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ERRORS IN SAMPLING

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  1. ERRORS IN SAMPLING Lesson #10 OCTOBER 5/6, 2011

  2. Errors in Sampling • Sampling Errors • Random sampling error – expressed as the margin of error.

  3. Errors in Sampling • Sampling Errors • Bad sampling methods – voluntary response samples, convenience samples.

  4. Errors in Sampling • Sampling Errors • Under coverage – happens when some groups in the population are not included in the sampling frame. (A sampling frame is a list of individuals from which a sample is drawn.) • What happens if under coverage occurs???

  5. Errors in Sampling • Non-Sampling Errors • Processing errors – mechanical errors, arithmetic errors, data entry errors

  6. Errors in Sampling • Non-Sampling Errors • Response errors – person lies about answer, remembers details incorrectly, doesn’t understand question

  7. Errors in Sampling • Non-Sampling Errors • Non-response errors – failure to obtain data from individual (refuses to cooperate or cannot be contacted) • This is becoming the biggest error category, which produces the most bias… (people don’t want to answer questions over the telephone; caller ID and answering machines prevent contact; and the elderly are often reluctant to participate)

  8. Errors in Sampling • Non-sampling errors • The wording of questions – if the question asks if the subject “favors” some policy as a means to an end, it is a loaded question that draws positive responses from subjects who are worried.

  9. Errors in Sampling • Example: “Do you favor imposing a closed campus for Irvington students if it means increased safety for your children?” • Example: “Do you favor leaving Irvington campus open for students during lunch time so that nearby businesses would not need to close due to lack of customers?”

  10. Wording Questions

  11. Other Types of Samples • Stratified Random Sample • Divide the sampling frame into distinct groups (strata). • Take SRS’s of each strata. • Example: Using the school’s population, the sampling frame can be divided into freshmen, sophomores, juniors, seniors, and staff (5 strata).

  12. Other Types of Samples • Cluster Sample • Can be used when the population falls into naturally occurring subgroups (like zip codes). • To select a cluster sample, divide the population into groups (clusters) and select all the members in one or more (but not all) the clusters. • Other examples: different periods of the same course, different branches of a bank, etc.

  13. Other Types of Samples • Systematic Sample • Each member of the population is assigned a number. The starting number is randomly selected, and then the sample members are selected at random intervals from the starting member. • Example: randomly chose the 15th person, then the interval was every 4th person after that, so the 15th, 19th, 23rd, 27th, and so on, were chosen for the survey. • Be careful using this in case there are regularly occurring patterns already in the population (every 4th person has a common quality).

  14. Questions to ask before you believe a poll • Who carried out the survey? • What was the population? • How was the sample selected? • How large was the sample?

  15. Questions to ask before you believe a poll • What was the response rate? • How were the subjects contacted? • When was the survey conducted? • What were the exact questions?

  16. So… • Government statistical offices answer these questions when they announce results; national opinion polls don’t always release the response rate (usually low). Newscasters usually leave out a lot of this information (boring); interest groups/newspapers don’t give this info because their methods are usually unreliable. • If someone doesn’t give all the information stated above, be skeptical!!!

  17. The most annoying word that is used in a conversation, according to a survey… Whatever!

  18. (from www.upi.com) • NEW YORK, Oct. 7 (UPI) -- Nearly half of Americans surveyed for a new poll said "whatever" was the most annoying word that could be used in a conversation. • The Marist Poll received votes for "You know" and "anyway," but they weren't even close to the 47 percent that pegged "whatever" as something that had definitely run its course. • The breakdown found "whatever" to be particularly tiresome among respondents in the Midwest, Latinos and people under 45 years old. • Phrases that bug Americans include "it is what it is" and "at the end of the day." • The poll, based on 938 responses, had a margin of error of 3.5 percent.

  19. Homework… • Page 110…114, #2.58-2.60, 2.62-2.66 • Bring a Gallup Poll for Friday’s group activity • Friday’s homework… Page 119…122, #2.68-2.71, 2.74

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