Body & HealthMental model
Without the natural course you can't spot an effect.
The natural history of a disease is the path it takes without treatment – from onset through the symptom-free phase to recovery, chronicity or death. If you don't know that path, you cannot tell a real effect apart from spontaneous improvement or coincidence.
Useful: That's why every efficacy trial needs a comparison group that isn't treated.
Source: en.wikipedia.org