A news article on the Reuters Health website reported that:
A study of over 1200 people over age 65 showed that the "owls" - those who tend to 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 . The researchers found that owls tend to have higher average income than the larks.
a. Is this an observational study or a randomized experiment?
b. What are the explanatory and response variables?