There is no way of being completely certain about the effects of therapy
fair comparisons of therapies provide the best basis for being confident about the effects of therapies.
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 therapy is used. For example, using PECs can help some autistic children express their preferences but it is not always successful or appropriate for every child.
BEWARE when someone suggests that it is 100% certain that a therapy works or is safe.
REMEMBER: There is some uncertainty about the effects of nearly all therapies.