Section 7.4 Cautions about Sampling

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Section 7.4 Cautions about Sampling

• Special Topics

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Undercoverage

• Sample surveys of large human populations require
  more than a good sampling design.
• We need an accurate and complete list of the
  population. Because such a list is rarely
  available, most samples suffer from some degree
  of Undercoverage.
• Undercoverage occurs when some groups in the
  population are left out of the process of
  choosing the sample.

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Nonresponse

• A more serious source of bias in most sample
  surveys is nonresponse.
• Nonresponse occurs when some groups in the
  population are left out of the process of
  choosing the sample. Either these individuals
  cant be contacted or refuse to answer.
• Nonresponse to sample surveys often reaches 50
  or more, even with careful planning and several
  callbacks. Because nonresponse is higher in urban
  areas, most sample surveys substitute other
  people in the same area to avoid favoring rural
  areas in the final sample. If the people
  contacted differ from those who are rarely at
  home or who refuse to answer questions, some bias
  remains.

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Response Bias

• Response Bias - for whatever reason the subject
  doesn't tell the truth.
• Often response bias occurs due to the choice of
  question or how it is worded. If a questions
  requires the admission of unpopular or illegal
  behavior in its answer, a respondent is apt to
  lie.
• Sometimes a respondent will not give an accurate
  answer due to faulty memory.

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Wording Effects

• When a question is leading or confusing, it often
  causes inaccurate answers for respondents.
• Watch out when a question starts with given
  that or considering.
• Make certain the readability of your questions
  corresponds well to the abilities of the members
  of your sample.
• http//www.learner.org/vod/vod_window.html?pid152
  

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Sampling Bias

• Sampling Bias is bias which is a result of poor
  sampling techniques.
• Remember that you should choose a sample in such
  a manner as to give each person in your
  population the same chance to be picked.

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Homework

• Worksheet 7.4