1. 5.1C (5.1 Review) Cautions about Sample Surveys

2. Cautions  Undercoverage occurs when some groups in the population are left out of the process of choosing the sample.  Nonresponse occurs when an individual chosen for the sample cannot be contacted or does not cooperate.  The behavior of the respondent or of the interviewer can cause response bias in sample results.  The wording of questions can also influence the answers given to a sample survey.

3. Other factors to consider  Could a non-random sample still be representative of the population?  Yes- it is possible to pick a group that is “typical” but not as likely if it is not a random process.  In general, does a larger or a smaller random sample give more accurate results?  More results = closer to the (true) population values.  If the sampling method is biased, will taking a larger sample reduce the bias?  No- it just means you have a larger set of biased data. 

4. Exercise: Non-Sampling sources of Bias  1. Suppose that simple random samples of adult Americans are asked to complete a survey describing their attitudes toward the death penalty. Suppose that one group is asked, “Do you believe that the U.S. judicial system should have the right to call for executions?” whereas another group is asked, “Do you believe that the death penalty should be an option in cases of horrific murder?”  Would you anticipate that the proportions of “Yes” responses might differ between these two groups? Explain.

5.  2. Suppose that simple random samples of students at the local community college are questioned about a proposed policy to ban smoking in all campus buildings. If one group is interviewed by a person wearing a t-shirt and jeans and smoking a cigarette whereas another group is interviewed by a nonsmoker wearing a business suit, would you expect that the proportions declaring agreement with the policy might differ between these two groups? Explain.

6.  3. Suppose that MHS administration wanted to interview random samples of students and ask if you have ever consumed an alcoholic beverage. Would you expect the proportion of “yes” responses to be close to the actual proportion of students who have engaged in underage drinking? Explain.

7.  4. Suppose that simple random samples of adult Americans are asked whether or not they approved of the president’s handling of foreign policy. If one group is questioned prior to a nationally televised speech by the president on his foreign policy and another is questioned immediately after the speech, would you be surprised if the proportions of people expressing approval differed between these two groups? Explain

8. Sources of bias  Together lets list four sources of bias that can affect sample survey results even if the sampling procedure used is indeed a random one. Our lists will be based on the preceding four questions.

9. Assignment Read pg. 290-303 5.1C pg. 283 #5.14, 5.15, 5.17, 5.20, 5.21, 5.30