Melioidosis, an infectious disease caused by a Gram-negative bacillus, is difficult to cure and treatment includes intravenous medications followed by oral medications for almost 14 weeks.  In a multicenter, double-blind, non-inferiority, randomized controlled trial published in The Lancet, 626 patients with culture-confirmed melioidosis who had received a course of parenteral antimicrobial drugs were randomized to take either (trimethoprim-sulfamethoxaxole) TMP-SMX plus placebo or TMP-SMX plus doxycycline for 20 weeks.  No difference was found between both groups in terms of the culture-confirmed recurrent melioidosis as well as the incidence of overall recurrent melioidosis and overall mortality.  Fewer patients reported adverse reactions in the TMP-SMX plus placebo group and this regimen was more preferable by patients. The authors concluded that TMP-SMX was not inferior to TMP-SMX plus doxycycline as an oral treatment for melioidosis. What is the difference between this type of study design and a superiority study?