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 their effects. However, it is rarely, if ever, possible to be 100% certain about the size of the effects or to know exactly what will happen if an intervention is used.
The environment is highly variable from place to place and time to time and many factors may cause the effect of an intervention to vary. Therefore predictions about the likely effects of an intervention are often highly uncertain.
BEWARE when someone suggests that it is 100% certain that an intervention works or is safe.
REMEMBER: There is some uncertainty about the effects of nearly all actions we take.