An April 9, 2002, News Release from the National Institutes

Short Answer

An April 9, 2002, news release from the National Institutes of Health describes a study that investigated whether the herb St. John's wort is effective for treating depression. An excerpt from this press release follows:
An extract of the herb St. John's wort was no more effective for treating major depression of moderate severity than placebo, according to research published in the April 10 issue of the Journal of the American Medical Association. The randomized, double-blind trial compared the use of a standardized extract of St. John's wort (Hypericum perforatum) to a placebo for treating major depression of moderate severity. The multi-site trial, involving 340 participants, also compared the FDA-approved antidepressant drug sertraline to placebo as a way to measure how sensitive the trial was to detecting antidepressant effects.
a. Is this an experiment or an observational study? (Do not bother to explain.)
b. Explain what it means for this study to be "double-blind" and also why this is an important component of the study design.