Reuters (June 24, 1997) reported on a study published in the Journal of the American Medical Association (1997, vol. 277, pp. 1940-1944) in which researchers recruited 276 volunteers (aged 18 to 55) and used nose drops to infect them with a cold virus. The volunteers were then quarantined and observed to see whether they came down with a cold or infection. They also answered questions about their social lives, including the number of different types of contacts they had (family, work, community, religious groups, and so on). Reuters reported that "those with one to three types of social contacts were 4.2 times as likely to come down with cold symptoms and signs of infection compared to those with six or more contacts."

a. Explain why this is an observational study, not a randomized experiment.

b. What were the explanatory and response variables of interest to the researchers?

c. The researchers in this study took an unusual step to help reduce the impact of confounding variables. Explain what they did and why it might have helped to reduce the impact of confounding variables.