As indicated under Search Strategy, the type of question, e.g. intervention, aetiology, prognosis, etc. will drive the research design and therefore in trying to answer a particular type of question, you will need to filter your search to appropriate study types.
The Evidence Pyramid was designed for locating studies of clinical effectiveness, i.e. finding out what works. These levels of evidence are most suited to intervention or therapy type questions. The top of the pyramid i.e. systematic reviews of clinical trials, represents a much higher strength of effectiveness compared to levels lower down the pyramid such as a primary study or expert opinion.
The pyramid illustrates a hierarchy of evidence, the best evidence appearing at the tip of the pyramid while at the bottom, the unfiltered literature which includes primary studies, expert opinion and grey literature. As you move up, more evaluation or filtering of studies has taken place as study designs become more rigorous and allow less bias. At the top of the pyramid we have systematic reviews and meta-analyses where a meticulous process has taken place to compare, evaluate and synthesize the results of numerous trials or studies. The systematic review of trials, which can include a meta-analysis, is often regarded as the ‘best evidence’.
For more information about the Levels of Evidence see the link below.