One of the first clinical trials: 12 sailors, 6 treatments.
In 1747, the Scottish ship's surgeon James Lind divided twelve scurvy patients aboard HMS Salisbury into six pairs and gave each pair a different remedy: cider, vinegar, seawater, dilute sulphuric acid, a spice paste, or two oranges and a lemon daily. Only the citrus group recovered within days. The Royal Navy still didn't mandate lemon juice until 1795.
Useful: A clean comparison persuades faster than a hundred anecdotes, even if institutions take decades.
Source: en.wikipedia.org