Unless an intervention is compared to groups without an intervention, it is not possible to know what would happen without the intervention, so it is difficult to attribute outcomes to the intervention.
For example, if a group of students improved at spelling when they started working in pairs, it might seem like the approach has helped. However, they may have improved at spelling anyway. Unless we compare the students that are working in pairs with those that aren’t we cannot know for certain if the intervention has had an influence.
BEWARE of claims that are only based on before and after comparisons and do not include a comparison group.
REMEMBER: Always ask what the comparisons are when considering claims about the effects of interventions. Claims that are not based on comparisons are not reliable.