The hierarchy of evidence
Before we start looking at different study designs let’s look at the pyramid opposite.
Different hierarchies exist for different question types and these are useful as they tell us what type of study you should look for when you first start searching for evidence to answer your question. Although there is no single universally accepted hierarchy of evidence there is a broad agreement on the relative strengths of the principal types of research studies.
For example if you look at the pyramid opposite and see that there are no relevant studies of the type at the top of the pyramid that would answer your question then you proceed to search for the next type of study in the list that could possibly answer your question.
If you look at the pyramid again you will see that the Randomised Control Trial (RCT) is ranked above observational studies, while expert opinion and anecdotal experience are ranked at the bottom. You will notice that the systematic review and meta-analysis (secondary research) is placed above the RCT. This is because the systematic reviews combine data from multiple RCTs.
So remember the higher up the hierarchy that a study comes, the more likely it is that the study can minimise the impact of bias on the results of the study. You can think of this in a slightly different way - the higher up the top of the hierarchy the more certain we are of the results.
The hierarchy of evidence is a core principle of Evidence Based Health Care because it ranks study types based on the strength and precision of their research methods.