A scientist is studying the effects of cell phone use on headache rate. She identifies 30 people who use cell phones and 30 who do not. Each of the 60 people is given a questionnaire designed to determine headache frequency. None of the 60 people who participated in the study knew they were part of a study. Which statement is true? a. This is a randomized comparative experiment.
b. This is an observational study.
c. This is a double-blind study.
d. This is a matched-pairs design.

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