The different types of study used in nutrition or clinical research can be grouped into “the hierarchy of evidence”, with case studies near the bottom and systematic reviews at the top. Although there is no single, universally-accepted hierarchy of evidence, there is broad agreement on the relative strength of the principal types of research, or epidemiological studies.
Systematic review and meta-analysis are ranked at the top, above experimental or intervention studies (eg. randomized clinical trials, RCTs), since these often combine data from multiple RCTs, and possibly from other study types as well. Intervention studies rank above observational studies, while expert opinion and case studies are ranked at the bottom.