The hierarchy of scientific evidence or the evidence pyramid:
The scientific evidence pyramid is a visual representation of quality of evidence in scientific studies.
The base of the triangle is the long but it is weak in its quality. Similarly, the number of sources that form poor quality evidence is higher than other types of study. i.e., the number of sources is large like the base.
The quality of the evidence increases as we move towards the top of the pyramid, while, the number of sources decreases.
Just below the pinnacle is the systematic review. Systematic review is the collection and synthesis of high quality evidence to a particular question.
At the pinnacle is the meta-analyses which a statistical integration of quantitative results to provide an estimated value. In other words, it is the analysis of analyses.
Meta-analyses, together with systematic reviews form the highest quality of evidence in scientific studies.