You divide 30 flu patients into three groups: one takes vitamin C, another takes Sambucus extract, and the third takes a placebo. That third group believes they are receiving a real treatment — and that belief matters. When you track symptoms across all three groups for 14 days, you are testing whether the extracts drive recovery or whether the act of believing you took something does the heavy lifting. If the placebo group recovers at a similar rate as the extract groups, the treatment itself may not be responsible for the improvement.