Question

independent practice
runners after a marathon commonly develop respiratory infections. researchers want to know if taking 500 milligrams of vitamin c daily will reduce infections. researchers randomly assigned 100 marathon runners to receive either vitamin c or placebo. all subjects were watched for 14 days after the big race to see if infections developed. the type of design used here?
randomized block
completely randomized
matched pairs
stratified

Explanation:

In a completely randomized design, subjects are randomly assigned to treatment groups. Here, 100 marathon runners are randomly assigned to either vitamin C or placebo (two treatment groups). There's no blocking (grouping based on a variable like age etc. as in randomized block), no pairing (as in matched pairs), and no stratification (dividing into subgroups/strata and then sampling).

Answer:

B. Completely Randomized