Let’s first look at experimental studies.
The primary study at the top of the pyramid is the Randomised controlled trial (RCT) and this study is considered the ‘Gold standard’.
RCTs are the most rigorous way of determining whether a cause-effect relation exists between the treatment (intervention) and outcome and for assessing the cost effectiveness of a treatment. Although these RCTs are powerful tools (if done properly) their use is limited by ethical and practical concerns.
There are some study designs that involve no formal randomisation in the allocation of intervention to subjects: these are referred to as quasi-experimental. These designs are frequently used to inform public health practice and they are used where randomisation is difficult to achieve. The problems with quasi experimental studies are that they cannot rule out the possibility that the association was caused by a third factor linked to both the intervention and outcome.