This study design pyramid shows the hierarchy of evidence-based literature. When searching for information, remember some evidence is stronger. Find and use the study designs towards the top of the hierarchy pyramid.
When you're determining which category an article falls under, there are a few things you should consider. The document below offers tips and examples for determining the level of evidence of articles in PubMed.
One way you can evaluate search results is by considering the relative importance of the journals in which the articles are published. The Journal Citation Reports database (listed as JCR Web of LHL's databases page) allows you to quickly look up a journal's impact factor (IP) and compare it to other journals in the same discipline or subject area. You can also view a ranked (by IP) list of journals in a subject area.
Systematic review
A review of a body of data that uses explicit methods to locate primary studies, and explicit criteria to assess their quality.
Meta-analysis
A statistical analysis that combines or integrates the results of several independent clinical trials considered by the analyst to be "combinable" usually to the level of re-analyzing the original data; also sometimes called pooling or quantitative synthesis.
Note: Both Systematic review and meta-analysis are sometimes called "overviews."
Randomized Controlled Trials
Individuals are randomly allocated to a control group and a group who receive a specific intervention. Otherwise the two groups are identical for any significant variables. They are followed up for specific end points.
Cohort Studies
Groups of people are selected on the basis of their exposure to a particular agent and followed up for specific outcomes.
Case Control
A study which involves identifying patients who have the outcome of interest (cases) and similar patients without the outcome of interest (controls), and looking back (retrospectively) to determine if they had the exposure of interest.
Case Series
A descriptive report on a series of patients with an outcome of interest. No control group is involved.
Case Study
A report based on a single patient or subject; sometimes collected together into a short series.
Expert Opinion and Anecdotal
A consensus of experience from the good and the great. Ex: Something someone told you after a meeting.
Several databases, including the 4 highlighted in this guide (PubMed, Google Scholar, CINAHL, & Scopus) show you how many times a given article has been cited by other articles. This can often be another indication of the relative importance of an article. (However, an article may be cited often for other reasons, such as for being controversial. So one cannot assume a highly cited article makes a worthwhile contribution to the literature.)