In a recent study, Z. Zhao and colleagues measured the levels of formaldehyde in the air in 34 classrooms in the schools in the city of Taiyuan, China. On the same day, they gave questionnaires to 1993 students aged 11–15 in those schools, asking them whether they had experienced respiratory problems (such as asthma attacks, wheezing, or shortness of breath). They found that the students in the classrooms with higher levels of formaldehyde reported more respiratory problems.

a. What is the outcome variable?

b. What is the treatment variable?

c. Was this a cohort study or a case-control study?

d. Was the study prospective, cross-sectional, or retrospective?

e. Could the results be due to confounding? Explain.