We can rarely, if ever, be 100% certain about the effects of interventions.
Fair comparisons of interventions 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 intervention effects or to know exactly what will happen if an intervention is used.
BEWARE when someone suggests that it is 100% certain that an intervention works or is safe/has no negative impacts.
REMEMBER: There is some uncertainty about the effects of nearly all interventions.