1. Section 7.4 Cautions about Sampling
Special Topics

2. 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.

3. 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 can’t 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.

4. 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.

5. 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.

6. 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.

7. Homework
Worksheet 7.4