Fair comparisons of interventions provide the best basis for being confident about the effects of an intervention.
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 approach or method is used with another group of students. This is because statistics work with probabilities and results are average effects across groups, so group effects may be different to the effect an intervention has on an individual.
BEWARE when someone suggests that he/she is 100% certain that an intervention works.
REMEMBER There is some uncertainty about the effects of nearly all interventions.