A study of the effectiveness of echinacea involved upper respiratory tract infections. One group of infections was treated with echinacea and another group was treated with placebos. The echinacea and placebo groups were determined through a process of random assignment (based on data from “Efficacy and Safety of Echinacea in Treating Upper Respiratory Tract Infections in Children” by Taylor et al., Journal of the American Medical Association, Vol. 290, No. 21).

Identify which of these types of sampling is used: random, systematic, convenience, stratified, or cluster.