We will consider a PM level of 17 or more to be high exposure, and a PM level of less than 17 to be low exposure.
1. How many people had high exposure to PM?
2. How many of the high-exposure people had wheeze symptoms?
3. What percentage of the high-exposure people had wheeze symptoms?
4. How many people had low exposure to PM?
5. How many of the low-exposure people had wheeze symptoms?
6. What percentage of the low-exposure people had wheeze symptoms?
7. Is there a large difference between the percentage of high-exposure people with wheeze symptoms and the percentage of low-exposure people with wheeze symptoms?
8. Explain why the percentage of high-exposure people with wheeze symptoms is the same as the percentage of school-return people with wheeze symptoms.
9. Explain why the percentage of low-exposure people with wheeze symptoms is the same as the percentage of mail-return people with wheeze symptoms.
10. As the weather gets warmer, PM goes down because wood stoves are used less. Explain how this causes the mode of response (school or mail) to be related to PM. I I. It is generally the case in epidemiologic studies that people who have symptoms are often eager to participate, while those who are unaffected are less interested. Explain how this may cause the mode of response (school or mail) to be related to the outcome.
12. Rather than send out questionnaires. the investigators could have telephoned a random sample of people over a period of days. Explain how this might have reduced the confounding.