Secondary research gathers and analyzes data focused on or around a specific question. Often this is a question that has been asked by several different researchers at different points in time, yet no clear consensus has been reached in the literature. Alternatively, this could be an investigation into the underlying methodologies, sources, definitions or assumptions in published work. Some examples:
Systematic reviews, meta-analyses, evidence syntheses, and other types of reviews are all referred to as secondary research. As shown in the image above, they are at the top of the hierarchy of evidence.