Strong evidence helps us understand what is effective and what is not effective. Strong evidence comes from good quality research and evaluation studies, replicated many times across different groups. To be sure something works, it needs to include a matched comparison group to help determine what would have happened without the intervention. This helps rule out the possibility that the results were down to something else.
Evaluations should also be repeated on different groups of people and in different contexts. This can help show how useful the findings are and whether they can be applied to different people and settings. Evaluations should also be repeated because the results from one study alone could be a fluke, due to something specific to that study, or something unique about the participants.