A news article on the Reuters Health website, December 18, 1998, reported that:
A study of over 1200 people over age 65 showed that the "owls"-those who go to sleep after 11 p.m. and rise after 8 a.m.-tend to be as healthy and intelligent as "larks"- those who go to bed before 11 p.m. and rise before 8 a.m.- according to a report in the December 19/26 issue of the British Medical Journal [Vol. 317, pp. 1675-1677]. The researchers also found that owls tend to have higher average income than the early birds.
a. Do you think this is based on an observational study or a randomized experiment? Explain.
b. It is mentioned that the "owls" have higher average income than the "larks" (early birds). Explain whether or not income is a possible confounding variable in this study.
c. Give an example of a possible confounding variable that is not mentioned.
d. Draw a figure similar to given Figure, illustrating the steps in this study.
