This design can be complicated, but requires fewer patients because subjects act as their own controls. A final way of looking at evidence is meta-analysis. A meta-analysis is a systematic and objective way of assembling data from many studies and arriving at a pooled estimate of treatment effect. When a meta-analysis is done well, it is often the quickest way for busy family doctors to get the best information in the shortest time. Given what has been said about matching question to method, there is a hierarchy of evidence that can be useful to delineate, especially when assessing effectiveness of treatment.
Most guidelines use the following scale to describe levels of evidence.
Some evidence from studies using such methods as quasi-experimental designs, time series, ecologic investigations, and case series, supports this intervention. It is not recommended in most circumstances. PDQ epidemiology .