A medical researcher wants to investigate the relationship between the duration of the common cold and the use of echinacea, vitamin C, or a combination of echinacea and vitamin C. The four treatments being studied are echinacea, 1000 mg of vitamin C, ehcinacea and 500mg vitamin C, and a control treatment where a subject receives a placebo. A simple random sample of size 120 is selected from a list of volunteers to make up the pool of experimental units. Each subject will record the length of the duration of any colds they have in a 1-year period. The 120 subjects are randomly assigned to the four groups and the subjects are given identical capsules containing their respective treatments in a fashion so that no subject will know which treatment they have received.

(a) Is this study an experiment or an observational study? Why?

(b) What are the treatments in this study?

(c) What are the experimental units in this study?

(d) What is the response variable in this study?

(e) Is this a blinded experiment?