We can rarely, if ever, be 100% certain about the effects of interventions.
Fair comparisons of treatments provide the best basis for being confident about the effects of interventions. However, it is rarely, if ever, possible to be 100% certain about the size of treatment effects or to know exactly what will happen if a treatment is used.
BEWARE when someone suggests that it is 100% certain that a treatment works or is safe.
REMEMBER: There is some uncertainty about the effects of nearly all interventions.