A study of cell phones and the risk of brain cancer looked at a
group of 469 people who had brain cancer. The investigators matched
each cancer patient with a person of the same sex, age, and race
who did not have brain cancer, and then asked about their use of
cell phones. Result: "Our data suggest that use of handheld
cellular telephones is not associated with risk of brain
cancer."
(a) What type of study is this?
Observational study: prospective
Observational study: retrospective
Observational study: cross-sectional
Experiment: matched pairs design
Experiment: randomized block design
Experiment: completely randomized design