A mineral supplement designed to prevent the flu was given to two groups of people during a scientific study. Dosages of the supplement were measured in milligrams per day, as shown in the
table below.
Supplement Dosages
Group
Dosage
(mg/day)
A
100
B
200
Afer 10 weeks, neither group reported a case of the flu. Which procedure would have made the
outcome of this study more valid?