Randomised intervention - i.e. randomly selecting a group of people to change their behaviour and comparing to the group that doesn’t change, who ideally receiv

Randomised intervention - i.e. randomly selecting a group of people to change their behaviour and comparing to the group that doesn’t change, who ideally receives a placebo. The random part is important, any other type of selection (e.g. observing people who choose to change their own behaviour) doesn’t work, in theory at least.